Fix assert in bakery_lock_release()

bakery_lock_release() expects an mpidr as the first argument however
bakery_lock_release() is calling it with the 'entry' argument it has
calculated. Rather than fixing this to pass the mpidr value it would be
much more efficient to just replace the call with

   assert(bakery->owner == entry)

As this leaves no remaining users of bakery_lock_held(), we might as
well delete it.

Fixes ARM-software/tf-issues#27

Signed-off-by: Jon Medhurst <tixy@linaro.org>