Revert "Merge branch 'wc-gitaly-keepalive-limit' into 'master'"

What does this MR do and why?

This reverts !73302 (merged). This setting causes long-running RPCs to shut down and causes the client to receive GOAWAY messages from the server. As explained in https://github.com/grpc/grpc/issues/25713, configuring Gitaly keepalive settings is dangerous: if the client sends too many, then the server will abruptly shut down the connection, causing RPCs to fail.

At the moment, it doesn't appear grpc-go has a way to configure or even disable this shutdown mechanism (I submitted https://github.com/grpc/grpc-go/pull/5162). For now, we should revert this change because it does more harm than good.

Go 1.13 should have enabled 15-second TCP keepalives by default; I'm not sure yet why this isn't working, or whether gRPC is fiddling with this as well.

Relates to #350580 (closed)

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