Correct the introduction comment, removing the native_posix mention,
and something which did not apply for several years.
Also remove an unnecessary include.
Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>
Including renaming the kconfig option
NATIVE_POSIX_TIMER into NATIVE_SIM_TIMER deprecating the first one.
Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>