powerpc: Remove useless config comment in asm/percpu.h

commit 0db880fc ("powerpc: Avoid nmi_enter/nmi_exit in real mode
interrupt.") has a config comment typo, and the #if/#else/#endif section
is small and doesn't nest additional #ifdefs so the comment is useless
and should be removed completely.

Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
Suggested-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240807025604.2817577-1-ruanjinjie@huawei.com