powerpc/syscalls: Remove __NR__exit

__NR__exit is nowhere used. On most architectures it was removed by
commit 135ab6ec ("[PATCH] remove remaining errno and
__KERNEL_SYSCALLS__ references") but not on powerpc.

powerpc removed __KERNEL_SYSCALLS__ in commit 3db03b4a

 ("[PATCH]
rename the provided execve functions to kernel_execve"), but __NR__exit
was left over.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/6457eb4f327313323ed1f70e540bbb4ddc9178fa.1629701106.git.christophe.leroy@csgroup.eu