SystemTools: Revert "Fix FileIsSymlink with Windows data deduplication"

This reverts !118 (merged). 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.