Remove composer/installer rule for drupal/core in root composer.json file

Problem/Motivation

In #2912387: Stop using wikimedia/composer-merge-plugin, an entry was added to the composer/installer rules to relocate the drupal/core project to the vendor directory:

    "extra": {
        "installer-paths": {
            "vendor/drupal/core": ["type:drupal-core"]
        }
    }

This entry was necessary to make drupal/core work correctly as a Composer path repository. However, there are two minor undesirable effects:

• The entry itself in composer.json is confusing, and documenting .json files is not convenient.
• A symlink is created from vendor/drupal/core to the core directory, which may cause some confusion.

Note that this is only done when doing Drupal core development; users who are working from a template project (which will include all Drupal tarball downloads) will use the drupal/core subtree split.

Proposed resolution

Remove this entry from installer-paths once it is no longer necessary.

Remaining tasks

Composer 1.9 has been released; this version includes a fix to allow path repositories to be relocated on top of their source directory in #8254

We must require that this version of Composer be used to do Drupal core development in order to remove this installer path. We need to decide when to do this.

Follow on work

The continuation issue is #2701253: [meta] Make it possible to put modules into the vendor directory

User interface changes

None.

API changes

None.

Data model changes

None.