Fix invalid style

There was a missing px unit after this.fixedHeaderHeight value
in CSS `calc` which seems to cause the side panel not appear
at all in Internet Explorer. `calc` has been removed completely
because it's not needed for this particular calculation where
both values have the same px unit.

Fixes learningequality#9100

kolibri/core/assets/src/views/FullScreenSidePanel/index.vue

@@ -171,7 +171,7 @@
       },
       closeButtonStyles() {
         return {
-          top: `calc((${this.fixedHeaderHeight} - 40px) / 2)`,
+          top: `${(this.fixedHeaderHeight - 40) / 2}px`,
         };
       },
       closeButtonFullScreenSidePanelStyles() {