drm/i915/execlists: Drop promotion on unsubmit

With the disappearance of NEWCLIENT, we no longer need to provide the
priority boost on preemption in order to prevent repeated gazumping,
and we can remove the dead code.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190515130052.4475-5-chris@chris-wilson.co.uk