Remove integration with AppVeyor

[morozov]

Integration with AppVeyor was originally implemented in #2617. This effort started in Jan 2017. At that time there was no CI for SQL Server in the DBAL project at all. Testing with SQL Server on Linux was added in #3050 (Mar 2018).

Why remove it

1. Builds on AppVeyor require maintenance. Despite the effort put into Fix PHP 8.4 installation on AppVeyor #7114 and Unconditionally update vcredist140 on AppVeyor #7117, the build still fails.
2. Oftentimes, AppVeyor is the slowest part of the build (it allows only one concurrent build).
3. There is no Windows-specific code in the source directory, so this won't impact test coverage.
4. I don't remember a single issue with SQL Server that would be found on AppVeyor or reproducible only on Windows.

[greg0ire]

Good riddance