Remove typehint on $LinkModel argument.

Adding a typehint here causes errors in a few userland plugins. We
should honor the previous interface as best we can.
Remove typehint from generateAssociationQuery()

While I think typehints are generally a good idea, this one broke a few
plugins, notably CakeDC/search. I don't think it is wise for 2.X release
to break BC in that way. I think reverting this change is probably the
safest path right now.
Merge pull request #3015 from cakephp/dbosource-bc-fix

2.5 - Dbosource bc fix