Made use of PathInfo.direct flag in trim_joins

Refs django#19385

Author: akaariai

django/db/models/sql/query.py

@@ -1445,8 +1445,7 @@ def trim_joins(self, target, joins, path):
  the join.
  """
  for info in reversed(path):
- direct = info.join_field == info.from_field
- if info.to_field == target and direct:
+ if info.to_field == target and info.direct:
  target = info.from_field
  self.unref_alias(joins.pop())
  else: