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.



git-svn-id: https://develop.svn.wordpress.org/trunk@27064 602fd350-edb4-49c9-b593-d223f7449a82