Trust xcrun now that we error out early for bad xcode-select paths

Since ae177adb2bd55ee5ad6367e7639c4cf0c774b63a, we can safely assume
that xcrun works, and a functioning xcrun will search dev_tools_path and
xctoolchain_path, so we can stop doing extra work here.

On CLT-only 10.7 and 10.8, xcrun will not work, but all the tools will
be in /usr/bin, which we check before invoking xcrun. Further, in this
case, dev_tools_path will be /usr/bin, and xctoolchain_path will not
exist, so the fallbacks here are unnecessary.