powerpc/8xx: Drop exception entries for non-existing exceptions

head_8xx.S has entries for all exceptions from 0x100 to 0x1f00.
Several of them do not exist and are never generated by the 8xx
in accordance with the documentation.

Remove those entry points to make some room for future growing
exception code.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/66f92866fe9524cf0f056016921c7d53adaef3a0.1576916812.git.christophe.leroy@c-s.fr