stream/rtsp: remove live555, nemesi, native rtsp implementations
Libavformat has rtsp support that is being actively developed. Remove
alternative implementations. Remove no longer needed build system
support for C++ files.
Libnemesi is dead. Native is unmaintained and probably won't work
better than libavformat. Live555 could still work better for some
streams, but it'd likely need maintenance because of upstream API
changes, and building the interface for it causes extra problems as
it's the only part of the player using C++. For those reasons it was
already disabled by default in commit
3f659fd1766 ("configure: disable
live555 by default") earlier.