#17968 disabled stats-based rebalancing because of unresolved issues under certain situations.

Re-enabling it wil involve:

• Fix behavior under splits without load (found while testing sky) storage: consider disabling kv.allocator.stat_based_rebalancing.enabled by default #17645
• Fixing storage: stats-based rebalancing can't handle large numbers of splits and scatters storage: stats-based rebalancing can't handle large numbers of splits and scatters #17671
• Fixing storage: Delayed update of per-store write stats can cause rebalance thrashing storage: Delayed update of per-store write stats can cause rebalance thrashing #17970
• Fixing storage: Consider which replica will be removed when adding a replica to improve balance storage: Consider which replica will be removed when adding a replica to improve balance #17971
• Fix the success criteria for the nightly allocator tests. Stats-based rebalancing means that we're no longer balancing just range counts, so the nightly allocator tests need a different set of metrics to measure. acceptance: stats-based rebalancing makes Allocator Test 3 to 5 10G flaky #17685
• Ideally, introducing an allocator acceptance test (a CI acceptance test rather than a nightly) that tests the allocator's behavior in various situations.