drm/xe: Remove obsolete async_ops from struct xe_vm

When sync binds were reworked and worker removed, async_ops became
obsolete. Remove it.

Fixes: f3e9b1f4 ("drm/xe: Remove async worker and rework sync binds")
Signed-off-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Francois Dugast <francois.dugast@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240117110908.2362615-1-mika.kuoppala@linux.intel.com