[SPARC64]: Kill no-remapping-needed code in head.S

It branches around some necessary prom calls, which we would
need to do even if we are mapped at the correct location already.
So it doesn't work.

The idea was that this sort of thing could be used for the eventual
kexec implementation, but it is clear that this will need to be
done differently.

Signed-off-by: David S. Miller <davem@davemloft.net>