powerpc/kuap: Remove unused inline function __kuap_assert_locked()

commit 2341964e ("powerpc/kuap: Remove __kuap_assert_locked()")
left behind this one, remove it.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220311130017.22936-1-yuehaibing@huawei.com