powerpc/64s/exception: remove 0xb00 handler

This vector is not used by any supported processor, and has been
implemented as an unknown exception going back to 2.6. There is
nothing special about 0xb00, so remove it like other unused
vectors.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190802105709.27696-22-npiggin@gmail.com