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>

include/drm/ttm/ttm_bo_driver.h

@@ -652,18 +652,6 @@ extern void ttm_tt_unbind(struct ttm_tt *ttm);
  */
 extern int ttm_tt_swapin(struct ttm_tt *ttm);
 
-/**
- * ttm_tt_cache_flush:
- *
- * @pages: An array of pointers to struct page:s to flush.
- * @num_pages: Number of pages to flush.
- *
- * Flush the data of the indicated pages from the cpu caches.
- * This is used when changing caching attributes of the pages from
- * cache-coherent.
- */
-extern void ttm_tt_cache_flush(struct page *pages[], unsigned long num_pages);
-
 /**
  * ttm_tt_set_placement_caching:
  *