routing: Remove unused rib_head members

[pouria]

Summary

Remove unused rnh_multipath and rib_algo_fixed members.
While here, convert rib_dying and rib_algo_init from uint32_t to bool.

[glebius]

sys/net/route/route_var.h

69–71

Practically a bool occupies one byte. Not sure if there is a standard about that, though. So change as is will take 2 bytes. But if you code it as bit fields, they are getting packed. With suggested change two bools will take only one byte and later you can add 6 more bools in the same manner without growing the structure.

I actually really would like us officially to promote this style instead of using pre-processor defines for flags.

[pouria]

Address @glebius comment