Remove legacy SQLite driver files

Remove the legacy, token-based MySQL-to-SQLite translator and its
supporting files. The new AST-based driver fully replaces it.

Removed files:
- class-wp-sqlite-translator.php
- class-wp-sqlite-lexer.php
- class-wp-sqlite-query-rewriter.php
- class-wp-sqlite-token.php
- class-wp-sqlite-pdo-user-defined-functions.php
- php-polyfills.php

Author: JanJakes