util: rm SilentTaskGroup. this does not seem to be needed anymore

I think this was originally needed due to incorrect management of group lifecycles,
which our current code is doing better.

also note that if we needed this, in newer aiorpcx, the name of
the field was ~changed from `_closed` to `joined`:
239002689a

electrum/network.py

@@ -45,7 +45,7 @@ from aiohttp import ClientResponse
 
 from . import util
 from .util import (log_exceptions, ignore_exceptions,
-                   bfh, SilentTaskGroup, make_aiohttp_session, send_exception_to_crash_reporter,
+                   bfh, make_aiohttp_session, send_exception_to_crash_reporter,
                    is_hash256_str, is_non_negative_integer, MyEncoder, NetworkRetryManager,
                    nullcontext)
 from .bitcoin import COIN
@@ -1184,7 +1184,7 @@ class Network(Logger, NetworkRetryManager[ServerAddr]):
 
     async def _start(self):
         assert not self.taskgroup
-        self.taskgroup = taskgroup = SilentTaskGroup()
+        self.taskgroup = taskgroup = TaskGroup()
         assert not self.interface and not self.interfaces
         assert not self._connecting_ifaces
         assert not self._closing_ifaces