Remove PostUploadPack RPC

For #4724 (closed)

In prior versions, we introduced Sidechannel protocols as an optimization to avoid transferring huge binary data over gRPC. This protocol was applied in *Sidechannel version of existing RPCs. We rolled out those RPCs and replaced all existing clients. There were no traffic to the old RPCs for a long time. It's time to wipe them out. Unfortunately, PostReceivePack is still used internally. PostUploadPack is the only removable RPC at the moment.

Some data to prove this endpoint is not used anymore:

• Prometheus metrics: https://dashboards.gitlab.net/d/000000199/gitaly-feature-status?orgId=1&refresh=5m&var-environment=gprd&var-method=PostUploadPack&var-prometheus=prometheus-01-inf-gprd&viewPanel=1&from=now-1d&to=now. Comparing to its Sidechannel version.
• Kibana logs: https://log.gprd.gitlab.net/goto/871dfa50-a139-11ed-9f43-e3784d7fe3ca. Comparting to its Sidechannel version.