Remove Protobuf-C++.podspec release.

This release has been broken since v4.22.x in 2022 with the addition of an abseil dependency (https://engdoc.corp.google.com/eng/doc/devguide/proto/news/2022-08-03.md#abseil-dep) and seems to have limited demand.

Users should use Protobuf C++ from our Github release instead: https://github.com/protocolbuffers/protobuf/releases

PiperOrigin-RevId: 694552734