Get rid of the typePrefix variable

[miselico]

At the moment, the schematree package contains the typePrefx variable. This is however only used during tree generation, for printing some info. Since this is specific to how we load data, it seems better to move this to move this closer to where the JSON dump is actually processed.

[miselico]

Note: this is more relevant wrt the work in https://github.com/martaannaj/RecommenderServer/tree/protobuffer_serialization