ERP5Type: Include OFSFolder2._cleanup method

  This aims to add compatibility with BTreeFolder2 API, even it is not
  required. Since some checkConsistency may call self._cleanup() regardless
  expecting that the folder is a [H]BTreeFolder2 always.

  This was detected when a post upgrade constrant was included to
  portal_categories
17 jobs for for_testrunner_3 in 0 seconds
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-Master

01:31:00

passed ERP5.CodingStyleTest-TestRunner3

12:14:44

failed ERP5.PerformanceTest-Master

00:35:06

failed ERP5.UnitTest-Master

03:24:41

failed ERP5.UnitTest-TestRunner3

16:39:06

passed SlapOS.Eggs.UnitTest-Master.Python3

02:06:13

passed Wendelin.UnitTest-Master

00:49:04

passed ERP5.CodingStyleTest-Master

01:01:33

passed ERP5.CodingStyleTest-TestRunner3

12:09:28

passed ERP5.CodingStyleTest-TestRunner3

01:07:57

failed ERP5.PerformanceTest-Master

00:34:59

passed ERP5.UnitTest-Master

02:19:32

failed ERP5.UnitTest-TestRunner3

10:40:34

failed ERP5.UnitTest-TestRunner3

01:35:22

passed SlapOS.Eggs.UnitTest-Master.Python3

02:03:33

passed SlapOS.Eggs.UnitTest-Master.Python3

02:04:17

passed Wendelin.UnitTest-Master

00:34:36