games/wargus: fix build with recent c++ library

In recent c++ library (such as on 14.x), <experimental/filesystem>
header would be removed in favor of standard <filesystem>. Remove
check for the former and don't do weird stuff when
experimental/filesystem is not present.

This is similar to 092787dffe45ea6ee1c5ea3b8cede34eea142992 for
games/stratagus

Reported by: pkg-fallout