onyx: Remove unreachable if condition

Both conditions are the same, so the second one is unreachable.

PR: 229550
Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: andreast, markj
Differential Revision: https://reviews.freebsd.org/D47167

(cherry picked from commit 6f96ef84b359137a51dc1e717887ca7d31ba7bad)