Bug 915535 - Remove directory tracking from build system. r=gps

It's not used except with pymake, anyways.
Bug 915535 - Remove subtier tracking from build system. r=gps

It's currently inaccurate, and soon, the very notion of subtier is going to
blow away in the compile/binaries targets.
Bug 988141 - Clean up config/recurse.mk after bug 969164. r=gps