logging: Drop confusing use of `propagate` in test_settings.

This line was added in e8ab7cd1a as a desperate measure to get the
`set_loglevel` helper to successfully suppress the `zulip.send_email`
logs.

In fact, the reason that just setting `level` there wasn't doing the
job had nothing to do with `propagate`, which doesn't interact with
`level`.  (See the long block comment in `zproject/settings.py`,
searching for "Python logging module", for my attempt at concisely
explaining these semantics.)  Rather, our setting for `level` was
getting clobbered by the use of `create_logger`; and setting
`propagate` to False worked by completely suppressing all logs to
`zulip.send_email` and descendants from reaching the normal handlers,
regardless of the log messages' levels.

Now that the `create_logger` issue is fixed (see a few commits before
this one), drop that.

zproject/test_settings.py

@@ -117,7 +117,6 @@ if not CASPER_TESTS:
 
     def set_loglevel(logger_name, level) -> None:
         LOGGING['loggers'].setdefault(logger_name, {})['level'] = level
-        LOGGING['loggers'].setdefault(logger_name, {})['propagate'] = False
     set_loglevel('zulip.requests', 'CRITICAL')
     set_loglevel('zulip.management', 'CRITICAL')
     set_loglevel('django.request', 'ERROR')