drm/ttm: drop unusued function declaration

This was removed in
f5a9a938 ("drm/ttm: remove TTM_MEMTYPE_FLAG_CMA")

but the the declaration was left dangling.
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200728045129.21065-1-airlied@gmail.com

include/drm/ttm/ttm_bo_driver.h

@@ -525,17 +525,6 @@ ttm_flag_masked(uint32_t *old, uint32_t new, uint32_t mask)
  * ttm_bo.c
  */
 
-/**
- * ttm_mem_reg_is_pci
- *
- * @bdev: Pointer to a struct ttm_bo_device.
- * @mem: A valid struct ttm_mem_reg.
- *
- * Returns true if the memory described by @mem is PCI memory,
- * false otherwise.
- */
-bool ttm_mem_reg_is_pci(struct ttm_bo_device *bdev, struct ttm_mem_reg *mem);
-
 /**
  * ttm_bo_mem_space
  *