Remove unneeded interchange SIGTERM handler (#3635)

This was introduced in PR #2629 to guard against the submit process
installing a SIGTERM handler and then that handler being unexpectedly
inherited by the interchange via multiprocesssing fork

PR #3463 changed the interchange to run as a fresh Python
process, which will not inherit SIGTERM handlers, so since
then this line has been vestigial.

Fixes issue #3588

parsl/executors/high_throughput/interchange.py

@@ -6,7 +6,6 @@
 import pickle
 import platform
 import queue
-import signal
 import sys
 import threading
 import time
@@ -313,16 +312,6 @@ def start(self) -> None:
         """ Start the interchange
         """
 
-        # If a user workflow has set its own signal handler for sigterm, that
-        # handler will be inherited by the interchange process because it is
-        # launched as a multiprocessing fork process.
-        # That can interfere with the interchange shutdown mechanism, which is
-        # to receive a SIGTERM and exit immediately.
-        # See Parsl issue #2343 (Threads and multiprocessing cannot be
-        # intermingled without deadlocks) which talks about other fork-related
-        # parent-process-inheritance problems.
-        signal.signal(signal.SIGTERM, signal.SIG_DFL)
-
         logger.info("Starting main interchange method")
 
         if self.hub_address is not None and self.hub_zmq_port is not None: