drivers: console: remove uart_mux and gsm_mux

Remove the deprecated uart_mux and gsm_mux modules and all of
their configurations/dependencies across zephyr.

Optimally uart_mux and gsm_mux would be removed in their own
respective PRs, but the two modules are directly coupled, so
to preserve bisectability, they must be removed together.

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