build: clean up some vestigial support code (NFC)
build: clean up some vestigial support code (NFC)

This was used for the parallel compilation of Swift code which has since
been migrated to CMake.  Remove the vestigial code to clean up the
module.

apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild