drivers: modem: gsm: remove mux_enabled flag and checks

mux_enabled check in the gsm_configure is unnecessary since it
is init and scheduled by gsm_ppp_start which means that the
mux must be disabled. The IS_ENABLED(CONFIG_GSM_MUX) check
should be good enough to determine whether or not the
mux_enable function should be ran.

Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>