Remove obsolete packages.config and update Appveyor config

[ivandrofly]

Deleted the outdated packages.config file from the project, as it's no longer required. Updated the Appveyor configuration to remove the related cache entry.

[ivandrofly]

Is there a way to just use dotnet restore without providing the package.config in appvoyer admin page?

See line 13 in the image

@niksedk

[ivandrofly]

Closing due to obsolete changes. Please ensure that the removal of src/ui/packages.config is still applied, as intended.

[ivandrofly]

https://github.com/SubtitleEdit/subtitleedit/pull/8950