Revert "Merge branch '36160-select2-dropdown' into 'master'"

This reverts merge request !14768

app/assets/stylesheets/framework/selects.scss

@@ -61,7 +61,6 @@
   border: 1px solid $dropdown-border-color;
   min-width: 175px;
   color: $gl-text-color;
-  z-index: 999;
 }
 
 .select2-drop.select2-drop-above.select2-drop-active {

changelogs/unreleased/36160-select2-dropdown.yml

----
-title: Decreases z-index of select2 to a lower number of our navigation bar
-merge_request:
-author:
-type: fixed