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/lnpeer.py

@@ -21,7 +21,7 @@ from . import bitcoin, util
 from . import ecc
 from .ecc import sig_string_from_r_and_s, der_sig_from_sig_string
 from . import constants
-from .util import (bh2u, bfh, log_exceptions, ignore_exceptions, chunks, SilentTaskGroup,
+from .util import (bh2u, bfh, log_exceptions, ignore_exceptions, chunks, TaskGroup,
                    UnrelatedTransactionException)
 from . import transaction
 from .bitcoin import make_op_return
@@ -105,7 +105,7 @@ class Peer(Logger):
         self.announcement_signatures = defaultdict(asyncio.Queue)
         self.orphan_channel_updates = OrderedDict()  # type: OrderedDict[ShortChannelID, dict]
         Logger.__init__(self)
-        self.taskgroup = SilentTaskGroup()
+        self.taskgroup = TaskGroup()
         # HTLCs offered by REMOTE, that we started removing but are still active:
         self.received_htlcs_pending_removal = set()  # type: Set[Tuple[Channel, int]]
         self.received_htlc_removed_event = asyncio.Event()