drm/i915/gem: Purge the sudden reappearance of i915_gem_object_pin()

This died many years ago as we now use i915_vma first and foremost.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191115170835.1367869-1-chris@chris-wilson.co.uk