Remove unused `DepthFirst` visitor

We only use `ToSQL` visitors in the our codebase, do not use
`DepthFirst` and `Dot` visitors.

The `DepthFirst` visitor (which was introduced at c86c37e5f) is used to
traverse an Arel (partial) ast with depth first.

Is there any worth to keep that undocumented feature with much code and
test cases.

This removes that unused `DepthFirst` code and test cases.

1 files changed, 7 insertions, 1 deletions

diff --git a/activerecord/test/cases/arel/visitors/dispatch_contamination_test.rb b/activerecord/test/cases/arel/visitors/dispatch_contamination_test.rb
index a07a1a050a..36f9eb49a2 100644
--- a/activerecord/test/cases/arel/visitors/dispatch_contamination_test.rb
+++ b/activerecord/test/cases/arel/visitors/dispatch_contamination_test.rb
@@ -48,7 +48,13 @@ module Arel
         node = Nodes::Union.new(Nodes::True.new, Nodes::False.new)
         assert_equal "( TRUE UNION FALSE )", node.to_sql
 
-        node.first # from Nodes::Node's Enumerable mixin
+        visitor = Class.new(Visitor) {
+          def visit_Arel_Nodes_Union(o); end
+          alias :visit_Arel_Nodes_True  :visit_Arel_Nodes_Union
+          alias :visit_Arel_Nodes_False :visit_Arel_Nodes_Union
+        }.new
+
+        visitor.accept(node)
 
         assert_equal "( TRUE UNION FALSE )", node.to_sql
       end