chore(cabal-install-solver): remove traceTree

[andreabedini]

Remove the traceTree debugging utility and all associated DEBUG_TRACETREE CPP machinery from
Distribution.Solver.Modular.Solver.

[geekosaur]

My question is, is this simply not being used or is there some way we ought to be using it. @sebright ?

[sebright]

When I commented on #11179, I meant that I thought that there should be an issue for discussing removing the tracetree feature. Why does it need to be removed? It seems like a relatively small amount of code for a feature that some people have found useful for understanding the structure of the search tree. It is especially useful for parts of the tree that aren't shown in the -v3 log.