refactoring: remove relay_restart_recovery as unneeded

As stated in its comment, this routine was needed back in the time when
raft followers ignored data coming from a non-leader. This is not the
case anymore. Now the followers accept the data from any source, so
there's no need to try and re-send some data which accumulated while the
node hasn't yet broadcast its leadership.

Part of #6754

NO_DOC=dead code removal
NO_CHANGELOG=dead code removal
NO_TEST=dead code removal