drm/ttm: remove stale declaration and field

Patch 649bf3ca77343e3be1e0af8e21356fa569b1abd9 has completely
removed ttm_backend structure. Remove lingering declaration
and related (now stale) field in ttm_tt structure,

CC: Jerome Glisse <jglisse at redhat.com>
Signed-off-by: Ilija Hadzic <ihadzic at research.bell-labs.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

include/drm/ttm/ttm_bo_driver.h

@@ -39,8 +39,6 @@
 #include "linux/fs.h"
 #include "linux/spinlock.h"
 
-struct ttm_backend;
-
 struct ttm_backend_func {
 	/**
 	 * struct ttm_backend_func member bind
@@ -119,7 +117,6 @@ struct ttm_tt {
 	unsigned long num_pages;
 	struct sg_table *sg; /* for SG objects via dma-buf */
 	struct ttm_bo_global *glob;
-	struct ttm_backend *be;
 	struct file *swap_storage;
 	enum ttm_caching_state caching_state;
 	enum {