[host_callback] Remove most of the jax.experimental.host_callback module

These APIs have been deprecated since March 2024 and they are subsumed by the new JAX external callbacks.
See #20385 for a discussion.

PiperOrigin-RevId: 682659677

CHANGELOG.md

@@ -16,6 +16,9 @@ When releasing, please add the new-release-boilerplate to docs/pallas/CHANGELOG.
   * {func}`jax.numpy.isscalar` now returns True for any array-like object with
     zero dimensions. Previously it only returned True for zero-dimensional
     array-like objects with a weak dtype.
+  * `jax.experimental.host_callback` has been deprecated since March 2024, with
+    JAX version 0.4.26. Now we removed it.
+    See {jax-issue}`#20385` for a discussion of alternatives.
 
 ## jax 0.4.34 (October 4, 2023)