[file] fix "rmtree" on non-dir path
[file] fix "rmtree" on non-dir path

Currently, "rmtree" on a non-directory path will attempt to recurse that
directory and remove it, resulting in an error if the target is a file
instead of a directory. For ease of use, enable "rmtree" to be called on
a single non-directory (file, link, etc.)

BUG=None
TEST=local
  - Ran the script on a file, no longer returns an error.

R=iannucci@chromium.org

Review-Url: https://codereview.chromium.org/2931343002

luci/recipes-py,luci/recipes-py