Remove deprecated `transaction_joinable=`

in favor of `begin_transaction`
with `:joinable` option.

activerecord/CHANGELOG.md

+*   Remove deprecated `transaction_joinable=` in favor of `begin_transaction`
+    with `:joinable` option.
+
+    *Arun Agrawal*
+
 *   Remove deprecated `decrement_open_transactions`.
 
     *Arun Agrawal*

activerecord/lib/active_record/connection_adapters/abstract_adapter.rb

@@ -387,12 +387,6 @@ def open_transactions
         @transaction.number
       end
 
-      def transaction_joinable=(joinable)
-        message = "#transaction_joinable= is deprecated. Please pass the :joinable option to #begin_transaction instead."
-        ActiveSupport::Deprecation.warn message
-        @transaction.joinable = joinable
-      end
-
       def create_savepoint
       end
 

activerecord/test/cases/transactions_test.rb

@@ -593,14 +593,5 @@ def test_transaction_isolation__read_committed
 
       assert_equal original_salary, Developer.find(1).salary
     end
-
-    test "#transaction_joinable= is deprecated" do
-      Developer.transaction do
-        conn = Developer.connection
-        assert conn.current_transaction.joinable?
-        assert_deprecated { conn.transaction_joinable = false }
-        assert !conn.current_transaction.joinable?
-      end
-    end
   end
 end