`kraken`: only do unsub if connected

Trying to send a message in the `NoBsWs.fixture()` exit when the ws is
not currently disconnected causes a double `._stack.close()` call which
will corrupt `trio`'s coro stack. Instead only do the unsub if we detect
the ws is still up.

Also drops the legacy `backfill_bars()` module endpoint.

Fixes #437

piker/brokers/kraken/feed.py

@@ -303,24 +303,6 @@ async def open_history_client(
         yield get_ohlc, {'erlangs': 1, 'rate': 1}
 
 
-async def backfill_bars(
-
-    sym: str,
-    shm: ShmArray,  # type: ignore # noqa
-    count: int = 10,  # NOTE: any more and we'll overrun the underlying buffer
-    task_status: TaskStatus[trio.CancelScope] = trio.TASK_STATUS_IGNORED,
-
-) -> None:
-    '''
-    Fill historical bars into shared mem / storage afap.
-    '''
-    with trio.CancelScope() as cs:
-        async with open_cached_client('kraken') as client:
-            bars = await client.bars(symbol=sym)
-            shm.push(bars)
-            task_status.started(cs)
-
-
 async def stream_quotes(
 
     send_chan: trio.abc.SendChannel,
@@ -419,6 +401,7 @@ async def stream_quotes(
             yield
 
             # unsub from all pairs on teardown
+            if ws.connected():
                 await ws.send_msg({
                     'pair': list(ws_pairs.values()),
                     'event': 'unsubscribe',