drm/nouveau: remove unused struct 'init_exec'

'init_exec' is unused since
commit cb75d97e

 ("drm/nouveau: implement devinit subdev, and new
init table parser")
Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Acked-by: Danilo Krummrich <dakr@redhat.com>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240517232617.230767-1-linux@treblig.org