fixes #4267

This is fairly unlikely to happen in practice

otherwise setLocalDescription and the arrival of first candidates might race
the rtpContentDescription being set

turns out we don’t need it and a better way is for RtpConnection to keep track of *all*
states in the current generation

using the parameter-free form of setLocalDescription() solves some potential race conditions
that can occur - especially once we introduce restartIce()

fixes #4215