[POWERPC] Fix bogus paca->_current initialization

When doing lockdep, I had two patches to initialize paca->_current
early, one bogus, and one correct.  Unfortunately both got merged
as the bad one ended up being part of the main lockdep patch by
mistake.  This causes memory corruption at boot.  This removes
the offending code.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>