Unused 'OwnPtr' hash traits

This patch removes the unused members and types from
'HashTraits<OwnPtr<P> >'. At the moment we do not
allow 'OwnPtr' hashing, moreover some of the removed
types are not compatible with the current implementation
of 'HashTableIterator'.

Review URL: https://codereview.chromium.org/188733002

git-svn-id: svn://svn.chromium.org/blink/trunk@168881 bbb929c8-8fbe-4397-9dbb-9b2b20218538