appveyor: Remove Meson job.
Appveyor Meson fails mysteriously some times, e.g.,
- https://ci.appveyor.com/project/mesa3d/mesa/builds/31780753
- https://ci.appveyor.com/project/mesa3d/mesa/builds/31857376
and now that we have msvc coverage on gitlab ci this is no longer necessary.
/cc @dstone @dbaker