drm/nouveau: bo: remove unused functions

nouveau_bo_new_pin_map() and nouveau_bo_unmap_unpin_unref() are unused,
hence remove them.

Reviewed-by: Ben Skeggs <bskeggs@nvidia.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240718165959.3983-3-dakr@kernel.org