Revert "ring: use turn.ring.cx as default TURN server"

We're facing a PJSIP issue when forcing a TURN server in case
of unavailability of this last.
So we prefer to revert this patch until we found a solution
for solving that.

This reverts commit bd2d9d27.

Reviewed-by: Alexandre Viau <alexandre.viau@savoirfairelinux.com>
Change-Id: I8ee88ae237a1ce56b74371cb513c7f98af1a5399