Remove a split test from the funnel.

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

• DELETE /funnels/{funnel_id}/split_test_steps/{id} (default branch=both)
  → removes the entire split test, both branches discarded.
• DELETE /funnels/{funnel_id}/split_test_steps/{id}?branch=left → removes
  the entire split test; the right 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}/split_test_steps/{id}?branch=right →
  removes the entire split test; the left branch's pages are
  promoted up under the split's parent.

See Destroying a split test in the Funnels Skill for how the collapse fits into the rest of the split-test surface.