Removed code signing and notarization from Windows/macOS builds

[n0toose]

The scripts in the scripts directory are flexible, in the sense that they
were also developed with scenarios, such as not being supplied any credentials
whatsoever. This means that we can just remove the environment variables
concerning code, and it should just work.

These changes have not been tested thoroughly yet, but are an improvement
given our situation. We hope to bring these back eventually, but they cost
money.

• I made sure the code compiles on my machine
• I made sure there are no unnecessary changes in the code
• I made sure the title of the PR reflects the core meaning of the issue you are solving
• I made sure the commit message(s) contain a description and answer the question "Why do those changes fix that particular issue?" or "Why are those changes really necessary as improvements?"

[n0toose]

This should resolve #9.

[Be-ing]

I recommend instead to modify the GitHub Actions workflow file to build when the environment variables are not specified. For example:
https://github.com/mixxxdj/mixxx/blob/main/.github/workflows/build.yml#L330

This allows for builds on the upstream repository to access the secrets for signing and notarization while also allowing GitHub Actions to run for pull requests from contributors' forks.

[n0toose]

I recommend instead to modify the GitHub Actions workflow file to build when the environment variables are not specified. For example:
https://github.com/mixxxdj/mixxx/blob/main/.github/workflows/build.yml#L330

That's a good idea, but we do want to move away from GitHub Actions, eventually, and we also don't have a reasonable way to test this. Wouldn't it be best to do this later down the line?

[Be-ing]

I'm confused... is this needed at all? CI builds are succeeding on the master branch and on pull requests...