drivers: modem: modem_cellular: Remove support for gsm_ppp

The new modem_cellular driver had support for the deprecated
generic zephyr,gsm-ppp dts compatible for backwards compatibility.

zephyr,gsm-ppp is being removed, and as such, modem_cellular.c
no longer needs to support it.

Signed-off-by: Bjarki Arge Andreasen <bjarki@arge-andreasen.me>