chore(ui): remove legacy deriveInitialStep and step-change error effect

deriveInitialStep (and its test) are dead now that the machine's initialState is
the sole authority for where the wizard mounts on (re)load; initialStepId no
longer lives on the context. The ResetCardErrorOnStepChange effect sentinel is
also gone — the submit runner clears the card error per submit.

Author: iagodahlem