Remove old-style (direct) TCP replication support

This issue has been migrated from [#11728](https://github.com/matrix-org/synapse/issues/11728).

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the old direct-to-master TCP replication protocol has been deprecated since [Synapse 1.18.0](https://github.com/matrix-org/synapse/blob/7fe7c454380a39a295aa77e75ab460975e9eb6a9/CHANGES.md#tcp-based-replication) (nearly a year and a half, as of this writing).

We still have a bunch of code to support it (`DirectTcpReplicationClientFactory` etc), and it's still mentioned in various bits of documentation (in particular https://matrix-org.github.io/synapse/develop/tcp_replication.html needs an update). Having multiple ways to do it is confusing and hard to maintain, so we should remove it properly.