rproxy: remove line context on backend disconnects

rproxy: remove line context on backend disconnects

It's worthless, especially when using `die' instead of `croak'.
In any case, there's not much we can do if upstream decides to
shut down a partially written stream for whatever reason (e.g.
OOM) so stop giving line information about it.  Adding request
information may be helpful at some point, but that's for another
time.