remove deprecated bits per django/django@8b1ff0d

Author: timgraham

django_mongodb/query.py

@@ -134,25 +134,17 @@ def get_cursor(self):
 
 def join(self, compiler, connection):
     lookup_pipeline = []
-    join_fields = self.join_fields or self.join_cols
     lhs_fields = []
     rhs_fields = []
     # Add a join condition for each pair of joining fields.
-    for lhs, rhs in join_fields:
-        if isinstance(lhs, str):
-            lhs_mql = lhs
-            rhs_mql = rhs
-        else:
-            lhs, rhs = connection.ops.prepare_join_on_clause(
-                self.parent_alias, lhs, self.table_name, rhs
-            )
-            lhs_mql = lhs.as_mql(compiler, connection)
-            rhs_mql = rhs.as_mql(compiler, connection)
-            # In the lookup stage, the reference to this column doesn't include
-            # the collection name.
-            rhs_mql = rhs_mql.replace(f"{self.table_name}.", "", 1)
-        lhs_fields.append(lhs_mql)
-        rhs_fields.append(rhs_mql)
+    for lhs, rhs in self.join_fields:
+        lhs, rhs = connection.ops.prepare_join_on_clause(
+            self.parent_alias, lhs, self.table_name, rhs
+        )
+        lhs_fields.append(lhs.as_mql(compiler, connection))
+        # In the lookup stage, the reference to this column doesn't include
+        # the collection name.
+        rhs_fields.append(rhs.as_mql(compiler, connection).replace(f"{self.table_name}.", "", 1))
 
     parent_template = "parent__field__"
     lookup_pipeline = [