Remove ClientSocketPoolManager::DumpMemoryStats()

The method does nothing any longer.

Bug: 1239513
Change-Id: I4b5cd791e60f8aea9ca3207160303ad6893a51cd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3118308


Reviewed-by: Maksim Orlovich <morlovich@chromium.org>
Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#915903}