UX: Remove unused strings (#19701)

* Remove unused strings
* Remove trailing quote from string
* Remove even more unused strings (they were removed in c4e10f2a9d)
* Don't use translations in tests which are only available on server
* Use more specific translation (and fix missing translation)

app/assets/javascripts/admin/addon/templates/customize-colors-show.hbs

@@ -106,7 +106,7 @@
                     }}
                     @action={{action "revert" c}}
                     @title="admin.customize.colors.revert_title"
-                    @label="revert"
+                    @label="admin.customize.colors.revert"
                   />
                   <DButton
                     @class={{concat
@@ -115,7 +115,7 @@
                     }}
                     @action={{action "undo" c}}
                     @title="admin.customize.colors.undo_title"
-                    @label="undo"
+                    @label="admin.customize.colors.undo"
                   />
                 {{/unless}}
               </td>
@@ -127,4 +127,4 @@
       <p>{{i18n "search.no_results"}}</p>
     {{/if}}
   </div>
-</div>
+</div>