drm/i915: Delete defunct i915_gem_request_assign()

i915_gem_request_assign() is not used since commit 77f0d0e9
("drm/i915/execlists: Pack the count into the low bits of the
port.request"), so remove the defunct code

References: 77f0d0e9

 ("drm/i915/execlists: Pack the count into the low bits of the port.request")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180102151235.3949-1-chris@chris-wilson.co.uk