Remove Marshal support from SchemaCache

YAML has been used to serialize the schema cache ever since 2016 with
Rails 5.1: 4c00c6e

Author: kaspth

activerecord/lib/active_record/connection_adapters/schema_cache.rb

@@ -103,16 +103,6 @@ def clear_data_source_cache!(name)
         @data_sources.delete name
       end
 
-      def marshal_dump
-        # if we get current version during initialization, it happens stack over flow.
-        @version = connection.migration_context.current_version
-        [@version, @columns, @columns_hash, @primary_keys, @data_sources]
-      end
-
-      def marshal_load(array)
-        @version, @columns, @columns_hash, @primary_keys, @data_sources = array
-      end
-
       private
 
         def prepare_data_sources

activerecord/test/cases/connection_adapters/schema_cache_test.rb

@@ -66,22 +66,6 @@ def test_clearing
         assert_equal 0, @cache.size
       end
 
-      def test_dump_and_load
-        @cache.columns("posts")
-        @cache.columns_hash("posts")
-        @cache.data_sources("posts")
-        @cache.primary_keys("posts")
-
-        @cache = Marshal.load(Marshal.dump(@cache))
-
-        assert_no_queries do
-          assert_equal 12, @cache.columns("posts").size
-          assert_equal 12, @cache.columns_hash("posts").size
-          assert @cache.data_sources("posts")
-          assert_equal "id", @cache.primary_keys("posts")
-        end
-      end
-
       def test_data_source_exist
         assert @cache.data_source_exists?("posts")
         assert_not @cache.data_source_exists?("foo")