[1.2.X] Fixed #14060 -- PostGIS never implemented the `~=` operator for...

[1.2.X] Fixed #14060 -- PostGIS never implemented the `~=` operator for geography types, so removed support for it.

Backport of r13757 from trunk.


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@13758 bcc190cf-cafb-0310-a4f2-bffc1f526a37