roachtest: drop legacy modes from rebalance_load matrix

**Summary:**
The roachtest `rebalance_load` ([pkg/cmd/roachtest/tests/rebalance_load.go:112](https://github.com/cockroachdb/cockroach/blob/master/pkg/cmd/roachtest/tests/rebalance_load.go#L112)) parameterizes its test matrix over multiple values of `kv.allocator.load_based_rebalancing`, including the legacy single-metric modes (`leases`, `leases and replicas`) alongside `multi-metric and count`.

Now that v26.3 makes MMA the default load-based rebalancer (via the new `auto` mode wired up in #169430), the legacy-mode rows in this matrix exist only to keep the dying single-metric path covered. We should drop the `count`/legacy flavor of the test once the MMA rollout is settled, retaining only the MMA mode(s) we actually want to keep covered going forward.

**Next Steps:**
- [ ] Trim `rebalance_load`'s mode matrix down to the MMA modes that are still meaningful.
- [ ] Adjust any mode-specific assertions accordingly.
- [ ] Consider whether the `count` flavor stays as a one-off coverage row or is removed entirely.

The legacy rows can come back temporarily if MMA rollout hits issues, but the steady state should not exercise the obsolete code path.

Epic CRDB-56265

Jira issue: CRDB-63449