Revert "Fixed 19895 -- Made second iteration over invalid queryset raise an exception too"

This reverts commit 2cd0edaa47.
This commit was the cause of a memory leak. See ticket for more details.
Thanks Anssi Kääriäinen for identifying the source of the bug.

django/db/models/query.py

@@ -104,7 +104,7 @@ class QuerySet(object):
             len(self)
 
         if self._result_cache is None:
-            self._iter = self._safe_iterator(self.iterator())
+            self._iter = self.iterator()
             self._result_cache = []
         if self._iter:
             return self._result_iter()
@@ -341,18 +341,6 @@ class QuerySet(object):
 
             yield obj
 
-    def _safe_iterator(self, iterator):
-        # ensure result cache is cleared when iterating over a queryset
-        # raises an exception
-        try:
-            for item in iterator:
-                yield item
-        except StopIteration:
-            raise
-        except Exception:
-            self._result_cache = None
-            raise
-
     def aggregate(self, *args, **kwargs):
         """
         Returns a dictionary containing the calculations (aggregation)