Drop use of `Symbol.brokers` everywhere

piker/data/feed.py

@@ -1635,7 +1635,7 @@ async def open_feed(
                     # apply `brokerd`-common steam to each flume
                     # tracking a symbol from that provider.
                     for fqsn, flume in feed.flumes.items():
-                        if brokermod.name in flume.symbol.brokers:
+                        if brokermod.name == flume.symbol.broker:
                             flume.stream = stream
 
                 assert len(feed.mods) == len(feed.portals) == len(feed.streams)

piker/ui/_display.py

@@ -1288,7 +1288,7 @@ async def display_symbol_data(
         hist_ohlcv: ShmArray = flume.hist_shm
 
         symbol = flume.symbol
-        brokername = symbol.brokers[0]
+        brokername = symbol.broker
         fqsn = symbol.fqsn
 
         hist_chart = hist_linked.plot_ohlc_main(

piker/ui/order_mode.py

@@ -292,7 +292,7 @@ class OrderMode:
             account=self.current_pp.alloc.account,
             size=0,
             symbol=symbol,
-            brokers=symbol.brokers,
+            brokers=[symbol.broker],
             oid='',  # filled in on submit
             exec_mode=trigger_type,  # dark or live
         )
@@ -709,7 +709,7 @@ async def open_order_mode(
 
         # load account names from ``brokers.toml``
         accounts_def = config.load_accounts(
-            providers=symbol.brokers
+            providers=[symbol.broker],
         )
 
         # XXX: ``brokerd`` delivers a set of account names that it