Removed call to Haystack backend setup method in backend wrapper

We want to avoid using the setup method when possible as it is broken
and makes three useless API calls.

LEARNER-2188

course_discovery/apps/edx_haystack_extensions/distinct_counts/backends.py

@@ -166,8 +166,9 @@ class DistinctCountsElasticsearchBackendWrapper(object):
         if len(query_string) == 0:
             return {'results': [], 'hits': 0, 'distinct_hits': 0}
 
-        if not self.backend.setup_complete:
-            self.backend.setup()
+        # NOTE (CCB): Haystack by default attempts to read/update the index mapping. Given that our mapping doesn't
+        # frequently change, this is a waste of three API calls. Stop it! We set our mapping when we create the index.
+        self.backend.setup_complete = True
 
         search_kwargs = self._build_search_kwargs(query_string, **kwargs)
         search_kwargs['from'] = kwargs.get('start_offset', 0)