powerpc: Cleanup asm-prototypes.c

Last call to sys_swapcontext() from ASM was removed by
commit fbcee2eb ("powerpc/32: Always save non volatile GPRs at
syscall entry")

sys_debug_setcontext() prototype not needed anymore since
commit f3675644 ("powerpc/syscalls: signal_{32, 64} - switch
to SYSCALL_DEFINE")

sys_switch_endian() prototype not needed anymore since
commit 81dac817 ("powerpc/64: Make sys_switch_endian() traceable")

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
[mpe: Keep _mcount() prototype to avoid modpost errors]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/3ed660a585df2080ea8412ec20fbf652f5bf013a.1646413435.git.christophe.leroy@csgroup.eu