No longer delete lines that have stopped running

Deleting the lines that have stopped running from the list of MuniLine
objects caused problems with lines that ended after midnight, if the
program was started when they weren't running, it would consider them to
have ended and never start checking predictions for them when they
started running. Deleting them from the list isn't really necessary for
performance, so it's easier to just keep all of the lines around for the
whole day.