Improve and clean up style-spec types

[mourner]

A part of #11426.

• Add Flow typing to previously untyped style-spec files (that are used by GL JS): deref.js, diff.js, empty.js, group_by_layout.js, validate_style.min.js
• Remove legacy constants validation from style-spec. Support for root style constants was removed at least 7 years ago, so it's no longer relevant and will simply be ignored from now on.
• Remove legacy declass.js as it's been unused for many years, and isn't even a part of the migrate function.
• Simplify empty.js that generates an empty style — it's still covered with tests so there's no need to be worried if we bump the style spec version.
• As a result of all these changes, reduce the gzipped bundle size by 214 bytes.

Launch Checklist

• briefly describe the changes in this PR
• write tests for all new functionality
• document any changes to public APIs
• manually test the debug page
• tagged @mapbox/map-design-team if this PR includes style spec API or visual changes
• apply changelog label ('bug', 'feature', 'docs', etc) or use the label 'skip changelog'