Fixed #14180 -- Prevented unneeded index creation on MySQL-InnoDB

Thanks zimnyx for the report and Simon Charette, Tim Graham for
the reviews.

docs/releases/1.8.txt

@@ -249,6 +249,9 @@ Database backends
   and up will support microseconds. See the :ref:`MySQL database notes
   <mysql-fractional-seconds>` for more details.
 
+* The MySQL backend no longer creates explicit indexes for foreign keys when
+  using the InnoDB storage engine, as MySQL already creates them automatically.
+
 Email
 ^^^^^