That is, leave only the code that was under USE_XDG. Previously the
Linux CMake build defined USE_XDG unconditionally, while it was not set
by the FreeBSD build.
The non-USE_XDG code was broken, and FreeBSD should follow the XDG
convention anyway (in particular, storing config files typically under
$HOME/.config/obs-studio). Defining USE_XDG in the os-freebsd.cmake
files would leave the non-USE_XDG code unused anywhere, so instead just
remove it.