Remove explicit phpunit/phpunit requirement

phpunit is not explicitly required, all the included tests run with
codeception, not phpunit (not directly, at least).

Fixes yiisoft#308

Author: particleflux

composer.json

@@ -23,7 +23,6 @@
         "yiisoft/yii2-debug": "~2.1.0",
         "yiisoft/yii2-gii": "~2.2.0",
         "yiisoft/yii2-faker": "~2.0.0",
-        "phpunit/phpunit": "~9.5.0",
         "codeception/codeception": "^5.0.0 || ^4.0",
         "codeception/lib-innerbrowser": "^4.0 || ^3.0 || ^1.1",
         "codeception/module-asserts": "^3.0 || ^1.1",