Remove everything to do with collision nodes from qdoc

Now that the qdoc link command has ability to tell qdoc which module contains a link target or whether to link to a QML or CPP entity, collision pages should no longer be necessary. In fact, qdoc hasn't been generating any collisions for some time. This task removes all the collision node code from qdoc.

qdoc: Add new checkForCollision()
But don't use it yet. qdoc will check for name
collisions within a module, once we decide what
collisions to look for and what to do about them.
This change puts a rewritten checkForCollision()
function back in, but doesn't use it yet.