WL#4094: Foreign keys: without indexes on the child table

Allow creation of foreign keys without automatic index creation
for the child table.
Or, allow dropping the automatically-created index
for the child table.
Either way, foreign keys will continue to work despite index absence.

From Peter's "WL#148 Note#1"
https://intranet.mysql.com/secure/mailarchive/mail.php?folder=4&mail=19697
"
Change#8: ...
4. If pk_column list is omitted, then pk table must have a PRIMARY KEY,
   just copy the column names from there. Try to support foreign
   key constraints if there is no key defined on the used tables.
"

From minutes of a Heidelberg discussions about foreign keys
https://inside.mysql.com/wiki/HD_Mtg:_Foreign_Keys_Workshop
"
Change#8: Agreed
Some participants said: We need *separate* WL entry for "support foreign keys
when there is no index on the child table."
Action item PeterG: Create that WL entry. 
"