Revert "socket-proxyd: Add --listener option for listener/destination pairs."

This reverts commit adcf4c81c58511b67644e17fa743d1729d3c9ccf.

We have a better solution for the problem of making two processes run in
the same namespace, and --listener is not needed hence and should be
dropped.

Conflicts:
	man/systemd-socket-proxyd.xml