refactor: drop superfluous return value
refactor: drop superfluous return value

Now that summary renderers always produce something, they no longer
need to indicate whether they did.

kinvolk/scope,alban/scope,weaveworks/scope,alban/scope,kinvolk/scope,alban/scope,kinvolk/scope,weaveworks/scope,kinvolk/scope,weaveworks/scope,weaveworks/scope,alban/scope,alban/scope,kinvolk/scope,weaveworks/scope,kinvolk/scope,weaveworks/scope,alban/scope