powerpc/64s: Remove unused exception code, small cleanups

This was not done before the big patches because I only noticed
them afterwards. It has become much easier to see which handlers
are branched to from which exception vectors now, and to see
exactly what vector space is being used for what.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>