Update rte autocomplete styling (#10503)

* amend className to reflect usage
* move new class into file used by Send/Edit composers
* add border styling to autocomplete

res/css/views/rooms/wysiwyg_composer/_SendWysiwygComposer.pcss

@@ -84,10 +84,3 @@ limitations under the License.
         border-color: $quaternary-content;
     }
 }
-
-.mx_SendWysiwygComposer_AutoCompleteWrapper {
-    position: relative;
-    > .mx_Autocomplete {
-        min-width: 100%;
-    }
-}