Micro optimise _callout_stop_safe() by removing dead code.

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