gccrs: remove unused callback substitution system

This was old code i used to badly figure out the generics on Index and
Iterator traits

gcc/rust/ChangeLog:

	* typecheck/rust-tyty-bounds.cc (TypeBoundPredicate::TypeBoundPredicate): remove.
	(TypeBoundPredicate::operator=): Likewise.
	(TypeBoundPredicateItem::get_tyty_for_receiver): Likewise.
	* typecheck/rust-tyty-subst.cc (SubstitutionArgumentMappings::error): Likewise.
	(SubstitutionArgumentMappings::empty): Likewise.
	(SubstitutionArgumentMappings::on_param_subst): Likewise.
	(SubstitutionArgumentMappings::get_subst_cb): Likewise.
	* typecheck/rust-tyty-subst.h (std::function<void): Likewise.
	* typecheck/rust-tyty.cc (ParamType::handle_substitions): Likewise.

Signed-off-by: Philip Herron <herron.philip@googlemail.com>