drm/ttm: remove declaration of ttm_tt_cache_flush

ttm_tt_cache_flush's implementation was removed in 2009 by commit
c9c97b8c, but its declaration has been hiding in ttm_bo_driver.h since
then.

It has been surviving in the dark for too long now ; give it the mercy
blow.

Reviewed-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>