drm/amd/amdgpu: Remove drm_debugfs_remove_files() call

drm_debugfs_cleanup() now removes all minor->debugfs_list entries
automatically, so no need to call drm_debugfs_remove_files().
Also remove empty drm_driver.debugfs_cleanup callback.

Cc: alexander.deucher@amd.com
Cc: christian.koenig@amd.com
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170126225621.12314-5-noralf@tronnes.org