Clean up: remove support for `mutate cst`

Motivation and Context

Mutation of CST is incomplete and broken, and not used. It was removed by mutation at AST level, as used by the testing framework.

Description

This MR drops support for CST mutation in CLI, and internally removes files which are not needed anymore.

Component

• compiler
• website
• webide
• vscode-plugin
• debugger

Types of changes

• Bug fix (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)
• Performance improvement (non-breaking change that improves performance)
• None (change with no changelog)

Changelog

Removed mutate cst sub-command (which was already previously disabled). Please use AST mutation from the testing framework.

Checklist:

• Changes follow the existing coding style (use dune @fmt to check).
• Tests for the changes have been added (for bug fixes / feature).
• Documentation has been updated.
• Changelog description has been added (if appropriate).
• Start titles under ## Changelog section with #### (if appropriate).
• There is no image or uploaded file in changelog
• Examples in changed behaviour have been added to the changelog (for breaking change / feature).