Micro optimise _callout_stop_safe() by removing dead code.

The CS_DRAIN flag cannot be set at the same time like the async-drain function
pointer is set. These are orthogonal features. Assert this at the beginning
of the function.

Before:

if (flags & CS_DRAIN) {
        /* FALLTHROUGH */
} else if (xxx) {
        return yyy;
}
if (drain) {
        zzz = drain;
}

After:

if (flags & CS_DRAIN) {
        /* FALLTHROUGH */
} else if (xxx) {
        return yyy;
} else {
        if (drain) {
                zzz = drain;
        }
}

Reviewed by: markj@
Tested by: callout_test
Differential Revision: https://reviews.freebsd.org/D26285
MFC after: 1 week
Sponsored by: Mellanox Technologies // NVIDIA Networking