powerpc: Remove now unused _TIF_RUNLATCH

'TIF_RUNLATCH' is already dropped from
commit fe1952fc

	powerpc: Rework runlatch code

So '_TIF_RUNLATCH' should be removed as well.

Signed-off-by: Tiejun Chen <tiejun.chen@windriver.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>