Since we dropped support of Vista and below in master toward 6.0,
those checks are needless. Removing the code that only worked in
older versions, and streamlining the resulting code.

Also, use kernel32.dll version for Windows version, instead of
deprecated GetVersionEx, and inconvenient VersionHelpers. Since both
GetVersion(Ex) and VersionHelpers (based on VerifyVersionInfo) are
subject to manifest-based behavior since Windows 8.1, this move will
hopefully result in more reliable OS version detection.

Change-Id: I3edd8fc1843e64b6a65bd3a126be6a085511f13c
Reviewed-on: https://gerrit.libreoffice.org/42905Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>
Tested-by: Mike Kaganski <mike.kaganski@collabora.com>