Remove Packages-arch-specific use from Soyuz

So, after having a long think about this, and many discussions surrounding it, I've come to the conclusion that Packages-arch-specific just isn't necessary anymore. The two most notable usecases for P-a-s are:

1) Not wanting to waste build time on slow arches where things haven't yet been ported:

Our slow arches have plenty of capacity to build (or fail) the world, and the inverse argument here is that restricting arches from P-a-s hides build failures from the FTBFS list and porters have no idea what needs working on. A failure log is a GOOD thing, in that it tells us what could be fixed/ported.

2) Arch-restrictions in source used to not work and not be respected by build infrastructres and tools:

This hasn't been true for a very long time, and for things that really are completely unportable and never will be ported (ie: nvidia drivers, packages whose sole raison d'etre are driving specific types of CPUs, etc), we can arch-restrict in source, and have no need to use P-a-s, and this is being done in most cases today.

There are plenty of spots, I'm led to understand, where soyuz doesn't always respect P-a-s today, and there are, as far as I can tell, no reasons to keep using it at all. Removing it would reduce complexity in a fair few areas, so this is something we should probably just suck up, tear it all out, QA, and rejoice.

NOTE: this is not about replacing P-a-s with some DB-driven madness, or improving on it, but removing it entirely. It's outlived its usefulness. Also, fixing this bug will obsolete https://bugs.launchpad.net/launchpad/+bug/669404