PassOwnPtr, PassOwnArrayPtr: const_pointer_cast should only remove 'const' qualifier.

Remove WTF smart pointers 'const_pointer_cast' functions as they are not used by anyone.
Another reason is that const_pointer_cast for PassOwnPtr and PassOwnArrayPtr was implemented wrongly (tried to cast to a different pointer type).

Review URL: https://codereview.chromium.org/25545002

git-svn-id: svn://svn.chromium.org/blink/trunk@158997 bbb929c8-8fbe-4397-9dbb-9b2b20218538