drm/i915/request: Remove the hook from await_execution

This was only ever used for FENCE_SUBMIT automatic engine selection
which was removed in the previous commit.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210708154835.528166-12-jason@jlekstrand.net