Deprecate empty AccessInterface and remove usages

I think #53 is a way to go

We could create more than one interface. For example:

/**
 * Access rules for a route.
 */
interface RouteAccessInterface {

	public function access(AccountInterface $account, RouteMatchInterface $route): AccessResultInterface;

}

Taking this a step further, in a routing file, if the class defined in _form implements this interface, then its ::access() method could be used automatically instead of having to be defined in _custom_access.

I updated the summary.

If we are not going to deprecate AccessInterface then the @todo in its code should be replaced with an explanation of when the interface should be used.

Before we get into patches, we need to discuss what to do.