Make sure we only create the minimum number of table indexes for MySQL.

This patch simplifies a bunch of code for all backends and removes some
duplicate index creation for MySQL, in particular (versions 4.x and later).
Patch from Nis Jørgensen.

Fixed #5671, #5680, #7170, #7186.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@7790 bcc190cf-cafb-0310-a4f2-bffc1f526a37