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/interface.py

@@ -45,7 +45,7 @@ from aiorpcx.jsonrpc import JSONRPC, CodeMessageError
 from aiorpcx.rawsocket import RSClient
 import certifi
 
-from .util import (ignore_exceptions, log_exceptions, bfh, SilentTaskGroup, MySocksProxy,
+from .util import (ignore_exceptions, log_exceptions, bfh, MySocksProxy,
                    is_integer, is_non_negative_integer, is_hash256_str, is_hex_str,
                    is_int_or_float, is_non_negative_int_or_float)
 from . import util
@@ -376,7 +376,7 @@ class Interface(Logger):
         # Dump network messages (only for this interface).  Set at runtime from the console.
         self.debug = False
 
-        self.taskgroup = SilentTaskGroup()
+        self.taskgroup = TaskGroup()
 
         async def spawn_task():
             task = await self.network.taskgroup.spawn(self.run())