Remove disused files

[jas88]

• .nuspec files no longer needed
• LGTM config obsolete, replaced by CodeQL

Proposed Changes

Summarise your changes, and include a link to the CHANGELOG diff.

Types of changes

What types of changes does your code introduce? Tick all that apply.

• Bugfix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that would cause existing functionality to not work as expected)
• Documentation-Only Update (if none of the other choices apply)
  • In this case, ensure that the message of the head commit from the source branch is prefixed with [skip ci]

Checklist

By opening this PR, I confirm that I have:

• Reviewed the contributing guidelines for this repository
• Ensured that the PR branch is in sync with the target branch (i.e. it is automatically merge-able)
• Updated any relevant API documentation
• Created or updated any tests if relevant
• Accurately updated the CHANGELOG
• Listed myself in the CONTRIBUTORS file 🚀
• Requested a review by one of the repository maintainers

Issues

If relevant, tag any issues that are expected to be resolved with this PR. E.g.:

• Closes Still using nuspec? #171