ci: remove broken appveyor config

The opam integration with appveyor has been broken for a while, so
builds fail with
```
+ curl -fsSL -o opam64.tar.xz https://github.com/fdopen/opam-repository-mingw/releases/download/0.0.0.2/opam64.tar.xz
+ tar -xf opam64.tar.xz
+ opam64/install.sh --quiet
warning: unzip not installed. opam will not work without it!
warning: patch not installed. opam will not work without it!
the following programs are not installed properly:
 patch unzip
I can't proceed :(
```

Signed-off-by: David Scott <[email protected]>

Author: djs55