units/serial-getty@.service: use the default RestartSec

For pluggable ttys such as USB serial devices, the getty is restarted
and exits in a loop until the remove event reaches systemd. Under
certain circumstances the restart loop can overload the system in a
way that prevents the remove event from reaching systemd for a long
time (e.g. at least several minutes on a small embedded system).

Use the default RestartSec to prevent the restart loop from
overloading the system. Serial gettys are interactive units, so
waiting an extra 100ms really doesn't make a difference anyways
compared to the time it takes the user to log in.