Remove unused bitemporal clauses

[wata727]

The valid_datetime returned by Relation#bitemporal_clause is propagated to child associations and affects the behavior of the bitemporal gem internals, but the valid_from, valid_to ​​do not appear to affect the behavior.

[wata727]

The following spec seems to be failing:

activerecord-bitemporal/spec/activerecord-bitemporal/bitemporal_scope_spec.rb

Lines 2220 to 2223 in d231d1a

	context "with Arel::Nodes::SqlLiteral" do
	let(:relation) { Blog.where(Blog.arel_table[:valid_to].gt(Arel::Nodes::SqlLiteral.new(1.days.to_s))) }
	it { is_expected.to include(valid_from: be_kind_of(Time), valid_to: nil) }
	end

Related commit: 601d966

Reading through this commit, it seems that this change will cause a breaking change to Relation#bitemporal_option. I'm not sure if this is actually useful, but it seems that the spec is that at least bitemporal_option contains valid_from/to that is resolved by ActiveRecord::Realtion.

For the reasons above, I'm going to discard this change. If you're interested, you can bump the major version and merge it.