TinyMCE: remove the 'spellchecker' plugin. It has been disabled for a while...

TinyMCE: remove the 'spellchecker' plugin. It has been disabled for a while and the back-end currently doesn't work. See #24067.
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git-svn-id: https://core.svn.wordpress.org/trunk@26837 1a063a9b-81f0-0310-95a4-ce76da25c4cd