roachtest/split: drop qps flavor of load-based split objective tests

**Summary:**
`runLoadSplits` ([pkg/cmd/roachtest/tests/split.go:799](https://github.com/cockroachdb/cockroach/blob/master/pkg/cmd/roachtest/tests/split.go#L799)) parameterizes its test cases over `kv.allocator.load_based_rebalancing.objective`, exercising both `qps` and `cpu`. This setting is read only by the legacy `StoreRebalancer` and has no effect under MMA.

MMA only ever balances on CPU, so the `qps` flavor of these tests is dead weight once MMA is the default (per #169430). Drop it; keep CPU coverage only.

**Next Steps:**
- [ ] Remove the `qps` arm of the load-split objective matrix in `split.go`.
- [ ] Drop or simplify the `setLoadBasedRebalancingObjective` helper if it loses its remaining caller.
- [ ] Sanity-check that any related assertions (e.g. expected split counts) still hold under CPU-only.

Epic CRDB-56265

Jira issue: CRDB-63451