Remove get_instance() from SimpleQueueClient

SimpleQueueClient was not thread-safe as the pika method
calls that we are using might disconnect and end up in a bad
state if two threads enqueue messages at the same time.

In order to avoid this, each user of the rabbitmq queue
gets its own instance of a SimpleQueueClient.

(imported from commit 694083b75cd58a60b8de282a8f40eb92a864c5ce)

zephyr/lib/queue.py

@@ -24,13 +24,6 @@ class SimpleQueueClient(object):
             credentials = pika.PlainCredentials(
                 'humbug', settings.RABBITMQ_PASSWORD))
 
-    @classmethod
-    def get_instance(cls):
-        # When subclassed, we will get one instance per subclass.
-        if not hasattr(cls, '_instance'):
-            cls._instance = cls()
-        return cls._instance
-
     def ready(self):
         return self.channel is not None