Remove PHPCS workflow and configuration file

It should have been removed in 69af948,
when the last php file inside lib was removed.

Author: greg0ire

.github/workflows/continuous-integration.yml

@@ -41,9 +41,6 @@ jobs:
       - name: "Install dependencies with composer"
         run: "composer update --no-interaction --no-progress --no-suggest"
 
-      - name: "Run squizlabs/php_codesniffer"
-        run: "vendor/bin/phpcs"
-
   validate-against-schema:
     name: "Validate against schema"