simplify finding of windows SDK in configure.ac

find_winsdk() is never called with a parameter

Change-Id: Ib0b0e5053a3bf792b66832d96ebf771251a24c1f
Reviewed-on: https://gerrit.libreoffice.org/13167
Tested-by: LibreOffice gerrit bot <gerrit@libreoffice.org>
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>