composer: Remove roave/security-advisories

Let's consider it later again, when we have had the chance to update our
dependencies properly.

Author: tdgroot

composer.json

@@ -37,7 +37,6 @@
     "require-dev": {
         "phpro/grumphp-shim": "^1.13",
         "phpunit/phpunit": "^8.5",
-        "roave/security-advisories": "dev-master",
         "squizlabs/php_codesniffer": "^3.7",
         "vimeo/psalm": "^4.26"
     },