Remove Windows support

[lengau]

What needs to get done

Remove Windows tests and other support items.

Why it needs to get done

Windows support is only partially tested and it's unclear how well it actually works.

[syncronize-issues-to-jira]

Thank you for reporting us your feedback!

The internal ticket has been created: https://warthogs.atlassian.net/browse/CRAFT-3232.

This message was autogenerated

[mr-cal]

Note that Windows support only had an entrypoint for snapcraft_legacy and therefor core20.

So this should be scheduled with the removal of core20 in Snapcraft 9.