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

drivers/gpu/drm/i915/i915_drv.h

@@ -1038,8 +1038,6 @@ int i915_gem_object_unbind(struct drm_i915_gem_object *obj,
 
 void i915_gem_runtime_suspend(struct drm_i915_private *dev_priv);
 
-int __must_check i915_gem_set_global_seqno(struct drm_device *dev, u32 seqno);
-
 int __must_check i915_gem_init(struct drm_i915_private *dev_priv);
 void i915_gem_driver_register(struct drm_i915_private *i915);
 void i915_gem_driver_unregister(struct drm_i915_private *i915);