[core] Revert async object pubsub posting for Serve throughput

[kiwoongyoon]

Title: [core] Revert async object pubsub posting for Serve throughput

Summary

This removes the PostingPublisher wrapper from core worker object pubsub publishing and returns object info publishing to direct pubsub::Publisher calls. The change targets the Serve single-replica throughput regression reported after #63983.

The borrowed-reference regression test added by #63983 is preserved; this patch only removes the async posting implementation and its now-unused Bazel target.

Test evidence

• git diff --check passed.
• python -m py_compile python/ray/tests/test_reference_counting_standalone.py passed.
• rg -n "PostingPublisher|posting_publisher" src/ray python/ray found no remaining references.
• git diff --exit-code -- python/ray/tests/test_reference_counting_standalone.py passed, confirming the correctness regression test remains unchanged.
• git diff --exit-code ecec4e589a -- src/ray/core_worker/BUILD.bazel src/ray/core_worker/core_worker_process.cc src/ray/pubsub/BUILD.bazel src/ray/pubsub/posting_publisher.h passed, confirming the C++/Bazel portion matches the upstream revert branch.

Risks or notes for maintainers

• I could not run Bazel C++ targets locally because Bazel/Bazelisk were unavailable in this worktree.
• I could not run Ray Python runtime tests because import ray failed in the local Python environment.
• I did not run Serve release microbenchmarks locally; the available performance evidence is the issue author's reverted-master benchmark data.
• This may trade off the Ray Data scheduler lock-contention improvement that [core] Post all object pubsub publishes to the IO thread to relieve lock contention #63983 was originally intended to provide.

Related issue: #64347

[gemini-code-assist]

Code Review

This pull request removes the PostingPublisher decorator class, its header file, and its associated build targets. The CoreWorkerProcessImpl now instantiates and uses pubsub::Publisher directly instead of wrapping it in a PostingPublisher. There are no review comments, and I have no feedback to provide.