Composer: remove explicit PHPCS dependency

The `squizlabs/php_codesniffer` package is a dependency of the `dms/coding-standard` package, not of **_this_** package.

Better to let the `dms/coding-standard` package (and its dependencies) manage the minimum require PHPCS version.

Author: jrfnl

composer.json

@@ -14,7 +14,6 @@
         }
     ],
     "require-dev": {
-        "squizlabs/php_codesniffer": "^3.4",
         "dms/coding-standard": "^9"
     },
     "autoload": {