Revert "Also install PHPUnit when running levels tests"

This reverts commit a909a52.

Author: ondrejmirtes

.github/workflows/tests.yml

@@ -155,9 +155,6 @@ jobs:
       - name: "Install dependencies"
         run: "composer install --no-interaction --no-progress"
 
-      - name: "Install PHPUnit 10.x"
-        run: "composer remove --dev brianium/paratest && composer require --dev --with-all-dependencies phpunit/phpunit:^10"
-
       - name: "Tests"
         run: "${{ matrix.script }}"