Remove "value" property definition from EntityFieldFetchItem

Problem/Motivation

The "value" property on the EntityFieldFetchItem field type is marked as being needed, but I saw this note on "\Drupal\Core\Field\FieldItemInterface" as guidance for implementing the "schema" method:

Computed fields having no schema should return an empty array.

I think it is worth a shot to try removing the value property since it has no functionality in that class.

Proposed resolution

1. Return an empty array in EntityFieldFetchItem::schema().
2. Remove the value property from EntityFieldFetchItem::propertyDefinitions().
3. Add an update hook that deletes the existing value column.

example code for #3:

$field_names = \Drupal::entityTypeManager()
                        ->getStorage('field_storage_config')
                        ->loadByProperties(['type' => 'entity_field_fetch']);

foreach ($field_names as $field_name) {
    $tables = [
        'node__' . $field_name, 
        'node_revision__' . $field_name
    ];

    foreach ($tables as $table) {
        if (Drupal::database()->schema()->fieldExists($table, $field_name . '_value')) {
            Drupal::database()->schema()->dropField($table, $field_name . '_value');
        }
    }
}

This might turn into more can-o-worms than it helps so maybe not worth the time. I tried to think of better ways to get rid of the value column in the update hook, but the way I proposed doesn't feel right...and I'm probably missing something that would cause errors.