Most of the toolbar items should be disabled for plain syntax

There's no point of using the toolbar in Plain syntax since it's not supported by the syntax by definition.

Reproduction step:

• Go to a translation page such as XWiki/AdminTranslations
• Click on edit

Obtained result:

• a WYSIWYG editor is opened (on a new page or in-place) with a toolbar, so you can for example type bold text, but this will never be saved

Expected result:

• the WYSIWYG editor should be opened (new page or in-place) without any toolbar.