rcu: remove duplicate CONFIG_RCU_CPU_STALL_DETECTOR

Impact: remove the old CONFIG_RCU_CPU_STALL_DETECTOR

tree_rcu introduce CONFIG_RCU_CPU_STALL_DETECTOR again.

These two are the same exactly except:

 the old one "depends on CLASSIC_RCU"
 the new one "depends on CLASSIC_RCU || TREE_RCU"

This patch remove the old one.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>