CI: use preinstalled sdk

[kbth]

Problem:

Packages requiring reboot:
 - vcredist140 (exit code 3010)
The recent package changes indicate a reboot is necessary.
 Please reboot at your earliest convenience.
Command exited with code 3010

Same as #2042.
Instead of explicitly installing the .NET SDK, this change uses the SDK preinstalled in the image.

The relevant line was originally added in the following PR to use .NET 9.0:
bd4f75b#diff-92ab9a36df5d8e9f7076f2fdec59492d1ac2d9cf27ea046767a7fc4d542ef3dc

On December 22, 2024, AppVeyor updated the Visual Studio 2022 image, and the included .NET SDK was upgraded to 9.0.101:
https://www.appveyor.com/updates/2024/12/22/

Therefore, these lines is no longer necessary.

[mgravell]

Ah, thanks; that was vexing me but I hadn't had time to dig. It kinda irks me when the image keeps changing underneath us - kinda defeats the purpose of a target image! I've overridden the CI change block - let's see how it does!

[mgravell]

merged with thanks