[Lens] fix drag and drop overlay (#162310)

## Summary

Fixes #160735

When implementing drag and drop for the first time, to move smoothly
between the main drop target and extra drop targets (duplicate or swap),
I created an extra overlay path that keeps the elements visible when
moving around (so when user drags out of the main drop target in the
direction of extra targets, we don't loose their visibility)

Turns out it started causing the 🔝 bug in the recent versions. 

This solution is not needed anymore though. Right now we keep elements
visible for a few seconds when user drags out no matter where the mouse
cursor is, so that's enough to go from main target to extra targets.

Here's the overlay path I removed (I added yellow transparent background
so it's more visible)
<img width="485" alt="Screenshot 2023-07-20 at 13 34 41"
src="https://github.com/elastic/kibana/assets/4283304/69acb1ff-1ada-4987-b47e-da282f8b992d">

packages/kbn-dom-drag-drop/src/drag_drop.tsx

@@ -628,21 +628,6 @@ const DropsInner = memo(function DropsInner(props: DropsInnerProps) {
 
   const mainTargetProps = getProps(dropTypes && dropTypes[0]);
 
-  const extraDropStyles = useMemo(() => {
-    const extraDrops = dropTypes && dropTypes.length && dropTypes.slice(1);
-    if (!extraDrops || !extraDrops.length) {
-      return;
-    }
-
-    const height = extraDrops.length * 40;
-    const minHeight = height - (mainTargetRef.current?.clientHeight || 40);
-    const clipPath = `polygon(100% 0px, 100% ${height - minHeight}px, 0 100%, 0 0)`;
-    return {
-      clipPath,
-      height,
-    };
-  }, [dropTypes]);
-
   return (
     <div
       data-test-subj={`${dataTestSubjPrefix}Container`}
@@ -658,32 +643,25 @@ const DropsInner = memo(function DropsInner(props: DropsInnerProps) {
         children={children}
       />
       {dropTypes && dropTypes.length > 1 && (
-        <>
-          <div
-            className="domDragDrop__diamondPath"
-            style={extraDropStyles}
-            onDragEnter={dragEnter}
-          />
-          <EuiFlexGroup
-            gutterSize="none"
-            direction="column"
-            data-test-subj={`${dataTestSubjPrefix}ExtraDrops`}
-            className={classNames('domDragDrop__extraDrops', {
-              'domDragDrop__extraDrops-visible': isInZone || activeDropTarget?.id === value.id,
-            })}
-          >
-            {dropTypes.slice(1).map((dropType) => {
-              const dropChildren = getCustomDropTarget?.(dropType);
-              return dropChildren ? (
-                <EuiFlexItem key={dropType} className="domDragDrop__extraDropWrapper">
-                  <SingleDropInner {...getProps(dropType, dropChildren)}>
-                    {dropChildren}
-                  </SingleDropInner>
-                </EuiFlexItem>
-              ) : null;
-            })}
-          </EuiFlexGroup>
-        </>
+        <EuiFlexGroup
+          gutterSize="none"
+          direction="column"
+          data-test-subj={`${dataTestSubjPrefix}ExtraDrops`}
+          className={classNames('domDragDrop__extraDrops', {
+            'domDragDrop__extraDrops-visible': isInZone || activeDropTarget?.id === value.id,
+          })}
+        >
+          {dropTypes.slice(1).map((dropType) => {
+            const dropChildren = getCustomDropTarget?.(dropType);
+            return dropChildren ? (
+              <EuiFlexItem key={dropType} className="domDragDrop__extraDropWrapper">
+                <SingleDropInner {...getProps(dropType, dropChildren)}>
+                  {dropChildren}
+                </SingleDropInner>
+              </EuiFlexItem>
+            ) : null;
+          })}
+        </EuiFlexGroup>
       )}
     </div>
   );

packages/kbn-dom-drag-drop/src/sass/drag_drop.scss

@@ -154,19 +154,12 @@ $reorderItemMargin: $euiSizeS;
   visibility: visible;
 }
 
-.domDragDrop__diamondPath {
-  position: absolute;
-  width: 30%;
-  top: 0;
-  left: -$euiSize;
-  z-index: $domDragDropZLevel0;
-}
-
 .domDragDrop__extraDropWrapper {
   position: relative;
   width: 100%;
   height: 100%;
   background: $euiColorLightestShade;
+  border-radius: $euiSizeXS;
 
   .domDragDrop__extraDrop,
   .domDragDrop__extraDrop:before {