Removing duplicate include of wplink plugin

This isn't urgent because the duplicate doesn't cause any issues, just cleanup.

The `wplink` plugin is already included above on [line 157](https://github.com/alleyinteractive/wordpress-fieldmanager/blob/f794acc83ffb985fd0546df12c0a9b030b4071f9/php/class-fieldmanager-richtextarea.php#L157).

The additional inclusion of `link` here was because [I misdiagnosed](https://github.com/alleyinteractive/wordpress-fieldmanager/pull/97) missing buttons as being from the missing plugin, whereas it seems that it had actually been from a problem in the trunk version of wplink which has since been fixed.