Remove unused PHPUnit dev dependencies from root composer.json

The root-level phpunit/phpunit and yoast/phpunit-polyfills packages
were only used by the legacy driver test suite, which was removed
along with the root phpunit.xml.dist and tests/ directory. The new
driver's tests live in packages/mysql-on-sqlite/ with their own
composer.json and dependencies.

Made-with: Cursor

Author: ashfame

composer.json

@@ -18,9 +18,7 @@
 		"squizlabs/php_codesniffer": "^3.7",
 		"wp-coding-standards/wpcs": "^3.1",
 		"phpcompatibility/phpcompatibility-wp": "*",
-		"php-parallel-lint/php-parallel-lint": "^1.3",
-		"yoast/phpunit-polyfills": "2.0.0",
-		"phpunit/phpunit": "8.5.52"
+		"php-parallel-lint/php-parallel-lint": "^1.3"
 	},
 	"config": {
 		"allow-plugins": {