Remove Sequel::SQL::Expression#to_s

This methods are not used any more internally since the dataset
literalization refactoring.  Most ruby objects have #to_s not
not require an argument, while these methods do.
Remove Dataset#table_ref_append

This private method is no longer used internally.
Remove methods in Dataset::PRIVATE_APPEND_METHODS

This methods are not used internally any longer.
Remove most methods in Dataset::PUBLIC_APPEND_METHODS

This methods are not used internally.  Most of them just exist
to support Sequel::SQL::Expression#to_s, which is also going away.

The three remaining methods (#literal, #quote_identifier, and
they cannot be removed.
Remove Dataset.def_append_methods

There's no reason this needs to be a method, since it is only
called with static input.  This will eliminate one of the
methods that allows code execution due to evaling strings.
Remove Model::InstanceMethods.class_attr_{overridable,reader}

These two methods allow for code execution via evaled strings,
and are only called with static input, so there isn't a reason
to have separate methods for them.