Remove unused gherkin grammar for syntax highlight

[kieran-ryan]

🤔 What's changed?

Removed unused grammar file for classic gherkin.

⚡️ What's your motivation?

• Source of confusion - easy to make assumption is in use.
• General tidy - removal of dead code - syntax highlighting is implemented through the Semantic Token Provider in the Language Service. Grammar file appears to have been introduced when creating the repository intially.
• Remains accessible through the archived cucumber-attic/cucumber-tmbundle repository

To verify is unused:

• Open the extension and use the Rule: keyword; it will highlight. As this keyword is not part of the earlier TextMate-defined grammar, you will not find a reference to it in that file - unlike other keywords.

• Check package.json. The grammars contribution endpoint is not set, like so (see VSCode docs):

  {
    "contributes": {
      "grammars": [
        {
          "language": "abc",
          "scopeName": "source.abc",
          "path": "./syntaxes/abc.tmGrammar.json"
        }
      ]
    }
  }

• Run the contributing Get Started guidelines with the syntaxes directory removed; syntax highlighting remains working.

🏷️ What kind of change is this?

• :bank: Refactoring/debt/DX (improvement to code design, tooling, documentation etc. without changing behaviour)

♻️ Anything particular you want feedback on?

📋 Checklist:

• [x] I agree to respect and uphold the Cucumber Community Code of Conduct