Bug 1891939: remove references to mce-tinymce class

when upgrading to TinyMCE 5, on commit ada3e2cd
the theme/raw/sass/lib/_tinymce.scss

the mce-tinymce class from the editor is changed to tox-tinymce

we should remove the references from the code to that class
in the jQuery selector and styles since they won't return any object.

behatnotneeded

Change-Id: Ia981e2c2599b4fb1ca6ade48bd376cca0cebbc85