Remove `$force` argument from `WP_Object_Cache::delete()`. From @nacin: "This was for internal-use-only, I don't think _deprecated_argument() is needed, but let's put a comment somewhere just so we know we'd have to be careful if we ever want to add a third argument here later."

`$force` argument has been removed, and docs were updated.

Fixes #22478.


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git-svn-id: http://core.svn.wordpress.org/trunk@26937 1a063a9b-81f0-0310-95a4-ce76da25c4cd