drm/ttm: revert "add optional LRU removal callback v2"

Without the custom LRU management the callback is not used any more.

agd: fix trivial warning

Reviewed-by: Sinclair Yeh <syeh@vmware.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-and-Tested-by: Roger.He <Hongbo.He@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>