Revert "[Bugfix] TGraph: Print out all nodes in dot output"

This reverts commit d74f1e1ae4087dd48f80a88c47afa32a4c5a1c3c.
Revert "Avoid linear searches in TGraph using Maps"

This reverts commit 841813310dc4c802d8e0b4c4096cb0b68b24a8ff.

Experiments show that this reorders constraints in a way that is
significantly worse for the SMT solving heuristics, and the (little)
time won on the faster lookup is lost several times over in the SMT
solver.