drm/i915: Remove unused i915_gem_context_lookup_engine

There are no known plans to start using it either.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190709093105.24699-1-tvrtko.ursulin@linux.intel.com