Remove the implementation of struct drm_driver.lastclose. The hook
was only necessary before in-kernel DRM clients existed, but is now
obsolete. The code in nouveau_vga_lastclose() is performed by
drm_lastclose().

v2:
- update commit description
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240812083000.337744-4-tzimmermann@suse.de