Remove the Spellchecker button from TinyMCE.

It appears Google has finally shut off the old private API it used. Spellcheckers built-in to browsers (and operating systems) seem to be the best alternative.

A WordPress plugin could still re-enable this TinyMCE plugin if they wanted.

props ryanduff for initial patch.
fixes #24789.



git-svn-id: https://core.svn.wordpress.org/trunk@24745 1a063a9b-81f0-0310-95a4-ce76da25c4cd