Remove composer-merge plugin

There was never any good reason to include this, since plugins are installed by

- composer: where dependencies become merged
- gateway: where dependencies are already included
- developers (A): include a custom repo in composer
- developers (B): run composer update in the plugin directory

It makes the system unpredictable and needlessly complex. Also there are reports of it slowing down composer

composer.json

@@ -30,8 +30,7 @@
         "october/system": "1.1.*",
         "october/backend": "1.1.*",
         "october/cms": "1.1.*",
-        "laravel/framework": "~6.0",
-        "wikimedia/composer-merge-plugin": "~2.0.1"
+        "laravel/framework": "~6.0"
     },
     "require-dev": {
         "phpunit/phpunit": "^8.4|^9.3.3",