Revert "Merge pull request #39759 from kamipo/marshal_load_legacy_ar_object"

This reverts commit 4fe1452534, reversing
changes made to 0f4258f646.

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 #39759.

I've added the constant alias in that time, but we usually don't
guarantee the marshalized object compatibility.

activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb

@@ -839,10 +839,6 @@ module ActiveRecord
           full_version_string.match(/^(?:5\.5\.5-)?(\d+\.\d+\.\d+)/)[1]
         end
 
-        # Alias MysqlString to work Mashal.load(File.read("legacy_record.dump")).
-        # TODO: Remove the constant alias once Rails 6.1 has released.
-        MysqlString = Type::String # :nodoc:
-
         ActiveRecord::Type.register(:immutable_string, adapter: :mysql2) do |_, **args|
           Type::ImmutableString.new(true: "1", false: "0", **args)
         end

activerecord/test/cases/base_test.rb

@@ -1301,21 +1301,6 @@ class BasicsTest < ActiveRecord::TestCase
     assert_equal 1, post.comments.length
   end
 
-  if current_adapter?(:Mysql2Adapter)
-    def test_marshal_load_legacy_6_0_record_mysql
-      path = File.expand_path(
-        "support/marshal_compatibility_fixtures/legacy_6_0_record_mysql.dump",
-        TEST_ROOT
-      )
-      topic = Marshal.load(File.read(path))
-
-      assert_not_predicate topic, :new_record?
-      assert_equal 1, topic.id
-      assert_equal "The First Topic", topic.title
-      assert_equal "Have a nice day", topic.content
-    end
-  end
-
   if Process.respond_to?(:fork) && !in_memory_db?
     def test_marshal_between_processes
       # Define a new model to ensure there are no caches