drivers: net: ppp: Remove integration with GSM_MUX

The gsm_ppp driver is deprecated, and will be removed, along with
its dependencies UART_MUX and GSM_MUX.

This commit removes the integration with GSM_MUX from ppp.c, making
it solely dependent on the chosen node zephyr,ppp-uart.

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