Drop dependency to phpunit because it is not used yet

Author: andrew-demb

composer.json

@@ -15,9 +15,6 @@
     "symfony/event-dispatcher": "^4.3|^5.0",
     "symfony/http-kernel": "^4.3|^5.0"
   },
-  "require-dev": {
-    "phpunit/phpunit": "@stable"
-  },
   "minimum-stability": "dev",
   "prefer-stable": true
 }