Remove no-op `PredicateBuilder::BaseHandler` constant

That is to prevent the "uninitialized constant" error when loading the
data marshalized by the previous version of Rails (Rails 6.0), it is not
needed for the future version of Rails.

See #39611.

I've added the no-op constant in that time, but we usually don't
guarantee the marshalized object compatibility.

This also avoids the "instance variable @future_result not initialized"
warning in the `test_marshal_load_legacy_relation`.

https://buildkite.com/rails/rails/builds/75166#3d7e57df-3679-4fa8-9f17-e1b0533c3ccf/1074-1081

activerecord/lib/active_record/relation/predicate_builder.rb

@@ -9,10 +9,6 @@ module ActiveRecord
     require "active_record/relation/predicate_builder/association_query_value"
     require "active_record/relation/predicate_builder/polymorphic_array_value"
 
-    # No-op BaseHandler to work Mashal.load(File.read("legacy_relation.dump")).
-    # TODO: Remove the constant alias once Rails 6.1 has released.
-    BaseHandler = BasicObjectHandler
-
     def initialize(table)
       @table = table
       @handlers = []

activerecord/test/cases/relation_test.rb

@@ -424,14 +424,6 @@ module ActiveRecord
       end
     end
 
-    def test_marshal_load_legacy_relation
-      path = File.expand_path(
-        "support/marshal_compatibility_fixtures/legacy_relation.dump",
-        TEST_ROOT
-      )
-      assert_equal 11, Marshal.load(File.read(path)).size
-    end
-
     test "no queries on empty IN" do
       assert_queries(0) do
         Post.where(id: []).load