pine64-pinephone: remove obsolete setup-modem.sh script

Since we now use the in-kernel modem driver of Megi's 5.9 kernel, the setup script has become unnecessary

https://megous.com/git/linux/tree/drivers/misc/modem-power.c?h=orange-pi-5.10#n531

https://megous.com/git/linux/tree/drivers/misc/modem-power.c?h=orange-pi-5.10#n580

https://megous.com/git/linux/tree/drivers/misc/modem-power.c?h=orange-pi-5.10#n610