[4.0]tinyMCE - unwanted CMS Content dropdown button

[wojsmol]

Pull Request for Issue #21711

Summary of Changes

CMS Content button in TinyMCE editor is only displayed when needed.

Testing Instructions

If you have a form with a "custom" editor field you see it. Otherwise edit the article form in administrator/components/com_content/forms/article.xml

• Change buttons="true" to buttons="false" for field-name articletext
• Open New Article
• The CMS Content drop-down button is there with an empty drop-down list.
  
  Example:

<field
      name="descripton" 
      type="editor" 
      ==>    buttons="false" 
      filter="JComponentHelper::filterText"
      height="200" 
     label="COM_MYCOMP_FIELD_DESCRIPPTION_LABEL"/>

Expected result

No button

Actual result

Button is there

Documentation Changes Required

N/A

cc @schnuti @dgrammatiko

This PR requires npm for testing.

[wojsmol]

closing in favour of the #21725