os: omit existence check for GetFinalPathNameByHandle

os: omit existence check for GetFinalPathNameByHandle

GetFinalPathNameByHandle exists since Windows Vista, which we no longer
support, so we don't need to prove that it exists before using it.

Updates #57003

Change-Id: Iff2bbe51d3baa3aabcaacf39ea3cbeda0088b9d7
Reviewed-on: https://go-review.googlesource.com/c/go/+/522195
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>