powerpc/xive: Remove unused inline function xive_kexec_teardown_cpu()

commit e27e0a94 ("powerpc/xive: Remove xive_kexec_teardown_cpu()")
left behind this, remove it.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200715025040.33952-1-yuehaibing@huawei.com