drm/i915: remove unused GEM_DEBUG_DECL() and GEM_DEBUG_BUG_ON()

There are already too many choices here, take away the unused ones.

Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220504183716.987793-1-jani.nikula@intel.com