Simplify tree_output config

The configuration was originally designed to support multiple output trees with different configs but this feature has never really been exploited. It created a few complications in case of configs with Analysis:bb𝛄𝛄 and Analysis:bb𝝉𝝉 with yaml includes + would interfere in the configuration of the output streams in the context of the developments towards one output file per channel in !1036 (merged)

So this functionality is removed here, with minimal adaptations to the minituple_cfg to potentially adjust it later if needed.