Remove a conditional split from the funnel.

A 1-branch conditional split is not a supported state, so any value
of branch collapses the entire split:

• DELETE /funnels/{funnel_id}/conditional_split_steps/{id} (default
  branch=both) → removes the entire conditional split, both branches
  discarded.
• DELETE /funnels/{funnel_id}/conditional_split_steps/{id}?branch=matched
  → removes the entire conditional split; the unmatched branch's
  pages are promoted up under the split's parent so they remain
  attached to the funnel's main workflow.
• DELETE /funnels/{funnel_id}/conditional_split_steps/{id}?branch=unmatched
  → removes the entire conditional split; the matched branch's
  pages are promoted up under the split's parent.