sched/ext: Fold balance_scx() into pick_task_scx()

With pick_task() having an rf argument, it is possible to do the
lock-break there, get rid of the weird balance/pick_task hack.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Tejun Heo <tj@kernel.org>