delete_dir_contents: Use remove_dir_contents
delete_dir_contents: Use remove_dir_contents

This is used for
 * In temp.rs, the "root directory" (ie .rustump/tmp)
 * In check_updates, on tmp and toolchains

We do not want to delete the containing directory, as discussed in
  https://github.com/rust-lang/rustup/issues/2430

There is still raw::remove_dir, which is used in a variety of places,
notably utils::remove_dir, some of the calls to which need fixing.

Signed-off-by: Ian Jackson <d68d4287c1430ca128ad85033356c9d259238c74@chiark.greenend.org.uk>

rust-lang/rustup,rust-lang/rustup,rust-lang/rustup,nodakai/rustup.rs,nodakai/rustup.rs,rust-lang/rustup,nodakai/rustup.rs,rust-lang/rustup,nodakai/rustup.rs,nodakai/rustup.rs