Remove dead spec

It only runs on Rails 5.1 or older, no longer supported.

spec/polyamorous/join_dependency_spec.rb

@@ -77,21 +77,5 @@ module Polyamorous
       specify { expect(subject.send(:join_root).drop(1)[1].table_name)
         .to eq 'comments' }
     end
-
-    context '#left_outer_join in Rails 5 overrides join type specified',
-            if: ActiveRecord::VERSION::MAJOR >= 5 && ActiveRecord::VERSION::MAJOR < 6 && ActiveRecord::VERSION::MINOR < 2 do
-
-      let(:join_type_class) do
-        new_join_dependency(
-          Person,
-          new_join(:articles)
-        ).join_constraints(
-          [],
-          Arel::Nodes::OuterJoin
-        ).first.joins.map(&:class)
-      end
-
-      specify { expect(join_type_class).to eq [Arel::Nodes::OuterJoin] }
-    end
   end
 end