chore: drop two dev scripts whose purpose ended with the rewrite

- `benchmark-jison.ts` — compared the legacy jison parser against the
  new pratt pipeline. Useful for the rewrite decision; now redundant
  with `benchmark.ts` (pratt vs `@csstools/css-calc`), and the script
  is coupled to the legacy `src/index.js` entry that's scheduled for
  removal at the next major.
- `check-issues.ts` — ran every open postcss-calc GitHub issue
  through the new pipeline to demonstrate the rewrite resolves the
  legacy complaints. PR-evidence tool; once the issues are triaged
  the script has no recurring purpose.

The remaining five scripts (`harvest-github`, `split-corpus`,
`show-divergences`, `randomizer`, `benchmark`) all have ongoing
maintenance value and stay.

Author: dkryaklin