Remove VS2012 support

With the next LAV Filters update, VS2012 support is gone in their side so this makes sense.

I kept the `COMPILER` variable in the batch files since it's useful to know what version is being used.

@Armada651: I assume you have a VS2013 copy, right? The rest of us already have VS2013 apart from @Underground78, but we should have this sorted soon-ish.

The only known issue is coverity v6.6.1, which is the latest at this point, fails with VS2013. I have already informed them, but I haven't got a reply.

Please, try the branch too; I only tried the lite build at this point.