mmaprototype: move doneShedding declaration into replica rebalancing block

Move the doneShedding variable declaration from the beginning of
rebalanceStore into the code block that contains the replica rebalancing
logic. This narrows the variable's scope to where it's actually used and
prepares for extracting this code into a separate method.

Author: tbg

pkg/kv/kvserver/allocator/mmaprototype/cluster_state_rebalance_stores.go

@@ -197,7 +197,6 @@ func (rs *rebalanceState) rebalanceStore(
 		store.StoreID, store.nls, store.sls, store.worstDim)
 	ss := rs.cs.stores[store.StoreID]
 
-	doneShedding := false
 	if true {
 		// Debug logging.
 		topKRanges := ss.adjusted.topKRanges[localStoreID]
@@ -238,6 +237,7 @@ func (rs *rebalanceState) rebalanceStore(
 
 	log.KvDistribution.VInfof(ctx, 2, "attempting to shed replicas next")
 	{
+		doneShedding := false
 		if store.StoreID != localStoreID && store.dimSummary[CPURate] >= overloadSlow &&
 			now.Sub(ss.overloadStartTime) < remoteStoreLeaseSheddingGraceDuration {
 			log.KvDistribution.VInfof(ctx, 2, "skipping remote store s%d: in lease shedding grace period", store.StoreID)