init_task: Remove redundant INIT_TASK_RCU_TREE_PREEMPT() macro

Back in the dim distant past, the task_struct structure's RCU-related
fields optionally included those needed for CONFIG_RCU_BOOST, even in
CONFIG_PREEMPT_RCU builds.  The INIT_TASK_RCU_TREE_PREEMPT() macro was
used to provide initializers for those optional CONFIG_RCU_BOOST fields.
However, the CONFIG_RCU_BOOST fields are now included unconditionally
in CONFIG_PREEMPT_RCU builds, so there is no longer any need fro the
INIT_TASK_RCU_TREE_PREEMPT() macro.  This commit therefore removes it
in favor of initializing the ->rcu_blocked_node field directly in the
INIT_TASK_RCU_PREEMPT() macro.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>