Removed the bulk of the sanity checking when merging QuerySets.

It was causing too many inconvenient and unintended problems when merging
legitimate subclasses (e.g. with geo-django classes). There's still a hook
there that we use when merging ValueQuerySets, but if you try to merge two
disparate QuerySets and it breaks all that happens is you get to keep both
pieces. We're no longer penalising the useful usages just for the people who
aren't concentrating.

Fixed #7113.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@7742 bcc190cf-cafb-0310-a4f2-bffc1f526a37