Delete non-public API jax.lib.xla_bridge._backends
This is doubly non-public: nothing under `jax.lib` is public, and also the object itself has a preceding underscore. Therefore it is safe to remove (chex had referenced this previously, but that's now addressed in https://github.com/google-deepmind/chex/commit/adaf1b2b7555e75a8ac118549e204520311f8ea0).
PiperOrigin-RevId: 704825268