fix(toolbar): remove ref to removed MAIN_TOOLBAR_BUTTONS (#2787)

react/features/toolbox/functions.web.js

  * is enabled, false - otherwise.
  */
 export function isButtonEnabled(name: string) {
-    return interfaceConfig.TOOLBAR_BUTTONS.indexOf(name) !== -1
-            || interfaceConfig.MAIN_TOOLBAR_BUTTONS.indexOf(name) !== -1;
+    return interfaceConfig.TOOLBAR_BUTTONS.indexOf(name) !== -1;
 }