Item12665:  Revert some of multi-tmce changes

Leave plugins by default in the pub/System/TinyMCEPlugin/plugins
directory,  no need to duplicate them.

If the plugin exists for the specific tinymce version, use it instead
of the default.  This will let development / testing of 4.x continue.

git-svn-id: http://svn.foswiki.org/trunk@17114 0b4bb1d4-4e5a-0410-9cc4-b2b747904278