Remove unused `left_joins_values` generation

This was added at #22125 but `left_joins_values` is never used.

activerecord/lib/active_record/relation.rb

@@ -2,7 +2,7 @@ module ActiveRecord
   # = Active Record \Relation
   class Relation
     MULTI_VALUE_METHODS  = [:includes, :eager_load, :preload, :select, :group,
-                            :order, :joins, :left_joins, :left_outer_joins, :references,
+                            :order, :joins, :left_outer_joins, :references,
                             :extending, :unscope]
 
     SINGLE_VALUE_METHODS = [:limit, :offset, :lock, :readonly, :reordering,

activerecord/test/cases/relation/mutation_test.rb

@@ -36,7 +36,7 @@ def relation
       @relation ||= Relation.new FakeKlass.new("posts"), Post.arel_table, Post.predicate_builder
     end
 
-    (Relation::MULTI_VALUE_METHODS - [:references, :extending, :order, :unscope, :select, :left_joins]).each do |method|
+    (Relation::MULTI_VALUE_METHODS - [:references, :extending, :order, :unscope, :select]).each do |method|
       test "##{method}!" do
         assert relation.public_send("#{method}!", :foo).equal?(relation)
         assert_equal [:foo], relation.public_send("#{method}_values")