mesa: remove no longer needed _mesa_is_bufferobj function

All buffers have Name != 0.

Note that there is no longer the pointer dereference to get Name,
so it's faster.

Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4466>
40 files changed