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
  #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 <ijackson@chiark.greenend.org.uk>

Author: ijackson

src/utils/utils.rs

@@ -637,7 +637,7 @@ where
 }
 
 pub fn delete_dir_contents(dir_path: &Path) {
-    match remove_dir_all::remove_dir_all(dir_path) {
+    match remove_dir_all::remove_dir_contents(dir_path) {
         Err(e) if e.kind() != io::ErrorKind::NotFound => {
             panic!("Unable to clean up {}: {:?}", dir_path.display(), e);
         }