MC-15083: Resolve performance issues

- Remove adminhtml tinymce.css completely

Author: davemacaulay

app/code/Magento/PageBuilder/view/adminhtml/web/css/source/form/element/_import.less

@@ -10,6 +10,5 @@
 @import '_container.less';
 @import '_map.less';
 @import '_margins-and-padding.less';
-@import 'tinymce.css';
 @import 'uploader/preview/_image.less';
 @import '_visual-select.less';