[#1667] shard: Drop shard pool

After adding an ops limiter, shard's `put` pool is redundant.

Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>

pkg/local_object_storage/engine/shards_test.go

@@ -17,7 +17,6 @@ func TestRemoveShard(t *testing.T) {
 	e, ids := te.engine, te.shardIDs
 	defer func() { require.NoError(t, e.Close(context.Background())) }()
 
-	require.Equal(t, numOfShards, len(e.shardPools))
 	require.Equal(t, numOfShards, len(e.shards))
 
 	removedNum := numOfShards / 2
@@ -37,7 +36,6 @@ func TestRemoveShard(t *testing.T) {
 		}
 	}
 
-	require.Equal(t, numOfShards-removedNum, len(e.shardPools))
 	require.Equal(t, numOfShards-removedNum, len(e.shards))
 
 	for id, removed := range mSh {