Remove php-coveralls/php-coveralls from composer.json

This is only relevant for CI (which is currently broken anyway). It will be
installed as part of the CI job in a follow-up commit.

Author: TimWolla

composer.json

@@ -22,8 +22,7 @@
         "php": "~7.1.0 || ~7.2.0 || ~7.3.0 || ~7.4.0 || ~8.0.0 || ~8.1.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^7.0 || ^8.0 || ^9.0",
-        "php-coveralls/php-coveralls": "~2.0"
+        "phpunit/phpunit": "^7.0 || ^8.0 || ^9.0"
     },
     "suggest": {
         "ext-gmp": "GMP is the preferred extension for big integer calculations",