drm/i915: remove unused i915_gem_set_global_seqno() declaration

The function was removed four years ago in commit 6faf5916
("drm/i915: Remove HW semaphores for gen7 inter-engine
synchronisation"). Finish the job.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/0f88380416c1bd457af58bb31aeb297749a6fb9e.1662390010.git.jani.nikula@intel.com