Cleanup SnapshotsInProgress for Readability (#71620)

* Formatting `SnapshotsInProgress` in a more reasonable way instead of mixing nested class, fields and methods.
* Removing pointless constants for the x-content serialization.
* Cleaning up double wrapping in `unmodifiableList`.
* Removing unused and incorrectly documented constructor from `SnapshotDeletionsInProgress`

server/src/main/java/org/elasticsearch/cluster/SnapshotDeletionsInProgress.java

@@ -68,14 +68,6 @@ private static boolean assertNoConcurrentDeletionsForSameRepository(List<Entry>
         return true;
     }
 
-    /**
-     * Returns a new instance of {@link SnapshotDeletionsInProgress} with the given
-     * {@link Entry} added.
-     */
-    public static SnapshotDeletionsInProgress newInstance(Entry entry) {
-        return new SnapshotDeletionsInProgress(Collections.singletonList(entry));
-    }
-
     /**
      * Returns a new instance of {@link SnapshotDeletionsInProgress} which adds
      * the given {@link Entry} to the invoking instance.