[EuiSearchBar] Remove IE from comment

- the fixed width affects modern browsers as well
- add note of Kibana findings for Emotion conversion

src/components/filter_group/_filter_group.scss

@@ -36,9 +36,11 @@
   display: flex;
 }
 
-// A fixed width is required in IE11 because of the shift in widths that can be caused
+// A fixed width is required because of the shift in widths that can be caused
 // by the loading animation that precedes the results.
 .euiFilterGroup__popoverPanel {
+  // Note for Emotion conversion: there are unfortunately 5+ direct
+  // usages of this className in Kibana that will need to be refactored
   width: $euiSize * 18;
 }