Remove leftover of wikimedia/composer-merge-plugin

Problem/Motivation

The wikimedia/composer-merge-plugin plugin was removed from core, see #2912387: Stop using wikimedia/composer-merge-plugin, But its config is in composer.json still.

Proposed resolution

Remove the config from composer.json.

-        "merge-plugin": {
-            "recurse": true,
-            "replace": false,
-            "merge-extra": false
-        },

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet