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.