stream: remove RTMP and RTMPDump dependency

[bastimeyer]

• drop RTMP stream implementation
• drop RTMP plugin
• drop RTMPDump dependency
• remove stream.streamprocess and utils.{rtmp,swf}
• remove "rtmp" from default stream types list
• remove "rtmp" from player-passthrough options list
• remove all --rtmp* CLI args and rtmp-* session options
• remove all --subprocess-* CLI args and subprocess session options,
  as these were used only by StreamProcess which is no longer needed
• update tests
• update docs and CLI argument help texts

---

ref #4040

This will require all users on Windows (who are using the Windows installer) to update their config files, as --rtmp-rtmpdump and its old deprecated parameter --rtmpdump have been removed, and those were set by default in the config file previously.

I'm not 100% satisfied with the rewritten docs yet in regards to the optional parameters, but this needs to be improved later, not now. I also had to remove one of the example parameters which I wanted to include in the rewritten docs section because of a bug in HTTPStream which I will fix afterwards (method=POST).

[back-to]

This will require all users on Windows (who are using the Windows installer) to update their config files, as --rtmp-rtmpdump and its old deprecated parameter --rtmpdump have been removed, and those were set by default in the config file previously.

help=argparse.SUPPRESS might be better instead of completely removing it
or ignore_unknown=True could be used.

[bastimeyer]

Suppressing could be done, but this will be a breaking change anyway, so I don't see a big issue with removing them now. Then there's also the issue with the related CLI args which were not set by the Windows installer config. Still a breaking change, but no intervention required.

I don't mind adding --rtmp-rtmpdump and --rtmpdump back and suppressing them (although it's kinda annoying). You decide.