Drop return value from `Filter::register_output`

Drop return value from `Filter::register_output`

This commit removes the return value from `Filter::register_output` as
creating a suitable value almost always entails blocking operations
(e.g., lookups via network request), which however conflicts with the
requirement that user calls should avoid blocking calls at all cost.

Removing the return value also rendered quite a bit of test code for
dependent transaction handling superfluous, which is therefore also
removed with this commit.