Remove Microsoft Visual Studio project files

We have had a build description to support native Windows compilation
under Microsoft Visual Studio 2015. Remove these in favour of the
cmake build, which is generally easier to maintain and is natively
supported by newer Windows build environments.

Also remove the corresponding appveyor ci configuration.

This project has a direct dependency on the `opus` library, which
made a similar removal starting with their 1.5 release a year ago.
This generally broke our Visual Studio build as well, since there
was no current release it could refer to within the project files.
As such, the appveyor ci jobs have been failing for some time.

Author: rillian