Remove useless argtype_inherit() code, and make consequent simplifications.

As I pointed out a few days ago, this code has failed to do anything useful
for some time ... and if we did want to revive the capability to select
functions by nearness of inheritance ancestry, this is the wrong place
and way to do it anyway.  The knowledge would need to go into
func_select_candidate() instead.  Perhaps someday someone will be motivated
to do that, but I am not today.