Drop support for pdo_dblib on Windows

Description

I just had a closer look at https://github.com/php/php-src/blob/master/ext/pdo_dblib/config.w32, and found it amazing that you can build the same DLL with two different names by just configuring --with-pdo-dblib and --with-pdo-mssql, if you manage to get the appropriate dependencies. That appears to be a problem, though, since FreeTDS claims to be a set of libraries for Unix and Linux, and to my knowlegde Sybase and Microsoft SQL Server are no longer compatible for decades. Furthermore, it occurs to me that pdo_sqlsrv is preferred to work with MS SQL Server for many years, and I can hardly imagine that anybody still uses pdo_mssql to connect to an SAP ASE (what it's apparently called since 2010).

For what it's worth, the official Windows builds neither contain php_pdo_dblib.dll nor php_pdo_mssql.dll for a long time.

So, can anyone shed some light on the state of pdo_dblib on Windows? Does anybody still use it? Can it even be built?

If not, I suggest to drop support for pdo_dblib on Windows altogether (i.e. remove config.w32 and the couple of Windows specific code in the C sources of the extension). And yes, this requires discussion on the internals and internals-win mailing lists, and possibly an RFC, but I wanted to gauge the feedback on this ticket first.

/cc @SakiTakamachi as codeowner of pdo_dblib

Start with ext/pdo_dblib/config.w32 and the Windows-specific C code in the extension, then review the issue discussion and the official Windows build contents. Confirm whether pdo_dblib can still be built or used on Windows and whether the proposed removal has the required internals or RFC agreement; done means an approved, validated decision and corresponding support changes.