[1.10.x] Fixed #26171 -- Made MySQL create an index on ForeignKeys with db_contraint=False.

Refactored "Prevented unneeded index creation on MySQL-InnoDB" (2ceb10f)
to avoid setting db_index=False.

Backport of 6bf7964023 from master

tests/indexes/models.py

@@ -19,6 +19,7 @@ class CurrentTranslation(models.ForeignObject):
 class ArticleTranslation(models.Model):
 
     article = models.ForeignKey('indexes.Article', models.CASCADE)
+    article_no_constraint = models.ForeignKey('indexes.Article', models.CASCADE, db_constraint=False)
     language = models.CharField(max_length=10, unique=True)
     content = models.TextField()