Never shield-wait `ipc_server.wait_for_no_more_peers()`

As mentioned in prior testing commit, it can cause the worst kind of
hangs, the SIGINT ignoring kind.. Pretty sure there was never any reason
outside some esoteric multi-actor debugging case, and pretty sure that
already was solved?

tractor/_runtime.py

@@ -1760,9 +1760,7 @@ async def async_main(
                 f'   {pformat(ipc_server._peers)}'
             )
             log.runtime(teardown_report)
-            await ipc_server.wait_for_no_more_peers(
-                shield=True,
-            )
+            await ipc_server.wait_for_no_more_peers()
 
         teardown_report += (
             '-]> all peer channels are complete.\n'

tractor/ipc/_server.py

@@ -814,10 +814,14 @@ class Server(Struct):
 
     async def wait_for_no_more_peers(
         self,
-        shield: bool = False,
+        # XXX, should this even be allowed?
+        # -> i've seen it cause hangs on teardown
+        #    in `test_resource_cache.py`
+        # _shield: bool = False,
     ) -> None:
-        with trio.CancelScope(shield=shield):
         await self._no_more_peers.wait()
+        # with trio.CancelScope(shield=_shield):
+        #     await self._no_more_peers.wait()
 
     async def wait_for_peer(
         self,