Drivers should all be cleaning up their memory managers
themselves now, so let the core just clean the system one up.

Remove the legacy cleaning interface.
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200804025632.3868079-36-airlied@gmail.com