drm/ttm: make TTM responsible for cleaning system only.

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