Remove auto-cleaning of the build directory

Remove auto-cleaning of the build directory

Over time this functionality has become obsolete through other means. Due to the
usage of `-L dependency=foo` it's not possible to pick up stale dependencies by
accident, and due to `--extern` you can only pick up a dependency. All of the
cases that auto-cleaning was fixing are now fixed through other methods, so
there's not much use ensuring a "clean build directory" any more.

This has the benefit of fixing issues like #961 where the downside of long
compiles outweighs the benefits of a "let's pretend we started from scratch"
build.

Closes #961