drm/i915: Remove unused for_each_uabi_class_engine

Unused macro after 99919be7 ("drm/i915/gem: Zap the i915_gem_object_blt code")
removed some code.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231102093248.362659-1-tvrtko.ursulin@linux.intel.com