Remove smartlist_choose_node_by_bandwidth()

We were only using it when smartlist_choose_node_by_bandwidth_weights
failed.  But that function could only fail in the presence of
buggy/ancient authorities or in the absence of a consensus.  Either
way, it's better to use sensible defaults and a nicer algorithm.
Merge remote-tracking branch 'public/bug13126'

Conflicts:
	src/or/or.h