prio_tree: remove unnecessary code in prio_tree_replace

Remove the code since 'node' has already been initialized in the begin of
the function

Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>