Remove HashMap::deleteAllKeys
https://bugs.webkit.org/show_bug.cgi?id=115806

Reviewed by Brent Fulgham.

This function was not used anywhere so let's get rid of it.

* wtf/HashMap.h:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@149742 268f45cc-cd09-0410-ab3c-d52691b4dbfc