TinyMCE:
- Restore the "link" button state to disabled by default and enabled when text or image is selected.
- Remove the (recently added) default `link` plugin, not needed for `wpLink`.
See #27309

git-svn-id: https://develop.svn.wordpress.org/trunk@27447 602fd350-edb4-49c9-b593-d223f7449a82