drm/xe/uapi: Remove DRM_XE_VM_BIND_FLAG_ASYNC comment left over

This is a comment left over of commit d3d76739
("drm/xe/uapi: Remove sync binds").

Fixes: d3d76739

 ("drm/xe/uapi: Remove sync binds")
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>