Revert "[runloop] Ensure timing arithmetic is done in ns."
Revert "[runloop] Ensure timing arithmetic is done in ns."

This shouldn't bite OpenBSD 7.1 and may fix regressions on Windows. The
underlying time code is still a little suspect, but this might be the
safest thing to do right now.

apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation