[PATCH V6 RESEND] sched/fair: Remove group imbalance from calculate_imbalance()

The group_imbalance path in calculate_imbalance() made sense when it was added back in 2007 with commit 908a7c1b9b80 ("sched: fix improper load balance across sched domain") because busiest->load_per_task factored into the amount of imbalance that was calculated. Beginning with commit dd5feea14a7d

[PATCH V6 RESEND] sched/fair: Remove group imbalance from calculate_imbalance()

The group_imbalance path in calculate_imbalance() made sense when it was added back in 2007 with commit 908a7c1b9b80 ("sched: fix improper load balance across sched domain") because busiest->load_per_task factored into the amount of imbalance that was calculated. Beginning with commit dd5feea14a7d