appveyor: remove Visual Studio 2013 build
appveyor: remove Visual Studio 2013 build

Because MairaDB 10.1.22 doesn't support it:

https://lists.launchpad.net/maria-discuss/msg04410.html

> Generally, we try to support 2 latest versions, which since couple of
> weeks means VS2015 and VS2017. As buildbot only builds 2015 now, it
> is easy to break the build on the old compiler and not to notice
> that.

mroonga/mroonga,mroonga/mroonga,mroonga/mroonga,mroonga/mroonga