Remove unnecesary classes to generate dropdowns

Since there are only one dropdown of each type there are no need to create a class to initialize multiple elements with the same CSS class name.

app/assets/javascripts/allowed_to_merge_dropdowns.js.es6

-class AllowedToMergeDropdowns {
-  constructor (options) {
-    const { $dropdowns, onSelect } = options;
-
-    $dropdowns.each((i, el) => {
-      new ProtectedBranchAccessDropdown({
-        $dropdown: $(el),
-        data: gon.merge_access_levels,
-        onSelect: onSelect
-      });
-    });
-  }
-}

app/assets/javascripts/allowed_to_push_dropdowns.js.es6

-class AllowedToPushDropdowns {
-  constructor (options) {
-    const { $dropdowns, onSelect } = options;
-
-    $dropdowns.each((i, el) => {
-      new ProtectedBranchAccessDropdown({
-        $dropdown: $(el),
-        data: gon.push_access_levels,
-        onSelect: onSelect
-      });
-    });
-  }
-}
\ No newline at end of file

app/assets/javascripts/protected_branch_create.js.es6

@@ -6,14 +6,16 @@ class ProtectedBranchCreate {
 
   buildDropdowns() {
     // Allowed to Merge dropdowns
-    new AllowedToMergeDropdowns({
-      $dropdowns: this.$wrap.find('.js-allowed-to-merge'),
+    new ProtectedBranchAccessDropdown({
+      $dropdown: this.$wrap.find('.js-allowed-to-merge'),
+      data: gon.merge_access_levels,
       onSelect: this.onSelect.bind(this)
     });
 
     // Allowed to Push dropdowns
-    new AllowedToPushDropdowns({
-      $dropdowns: this.$wrap.find('.js-allowed-to-push'),
+    new ProtectedBranchAccessDropdown({
+      $dropdown: this.$wrap.find('.js-allowed-to-push'),
+      data: gon.push_access_levels,
       onSelect: this.onSelect.bind(this)
     });