remove #2629 signal handler from interchange

[benclifford]

PR #2629 added a force of signal handler because it needs to deal with being launched by multiprocessing.fork. PR #3463 makes interchange into its own process, which should get default signal handlers anyway (?) and so I think there's no need for this additional complication.

cc @rjmello who has looked at interchange start/exit the most recently

[rjmello]

Your reasoning sounds right to me.