plus: remove handler once it has been consumed

edmond/plugins/plus.py

@@ -22,7 +22,7 @@ class PlusPlugin(Plugin):
         if not self.should_handle_command(event.arguments[0], no_content=True):
             return False
 
-        if handler := self.handlers.get(event.target):
+        if handler := self.handlers.pop(event.target, None):
             handler(event)
         else:
             self.signal_failure(event.target)