test,openbsd: remove superfluous ifdef guard (#4461)

The test is skipped in its entirety on OpenBSD so there is no point in
compiling out code on said platform later on, it's not run anyway.

test/test-udp-multicast-join.c

@@ -126,7 +126,7 @@ static void cl_recv_cb(uv_udp_t* handle,
     r = uv_udp_set_membership(&server, MULTICAST_ADDR, NULL, UV_LEAVE_GROUP);
     ASSERT_OK(r);
 
-#if !defined(__OpenBSD__) && !defined(__NetBSD__)
+#if !defined(__NetBSD__)
     r = uv_udp_set_source_membership(&server, MULTICAST_ADDR, NULL, source_addr, UV_JOIN_GROUP);
     ASSERT_OK(r);
 #endif