pinephone: setup-modem.sh has VoLTE related AT command
We used to have our own AT commands to configure the pinephone modem in pmaports. Then we transitioned to letting megi's modem-power manager configure it. Now we are about to adopt eg25-manager in !1815 (closed).
From the early days, we still have one VoLTE related AT command remaining. Let's get rid of the custom scripts and only use those from an upstream repository, so it is more consistent across distros.
@PureTryOut: you added the command in !1434 (merged) and reported that it works for you. Can you upstream it to eg25-manager, and verify that it works as expected?