This reverts commit ef373416 (SystemTools: Fix FileIsSymlink with
Windows data deduplication, 2019-01-18).  It regressed `FileIsSymlink`
such that it does not always recognize broken symlinks as symlinks.

Revert pending further investigation.  Notes for re-introduction:

* We need to add a test to cover the case that broke.
* We should save the Encoding::ToWindowsExtendedPath result and
  use it for both GetFileAttributesW and CreateFileW instead of
  using Encoding::ToWide for the latter.