erp5_core: Fix activity explosion bug.

Do not call container.reindexObject every time, otherwise the number
of activities explode because of recurseCallMethod.
6 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-Master

00:34:53

passed ERP5.PerformanceTest-Master

00:27:12

failed ERP5.UnitTest-Master

01:16:47

failed ERP5.UnitTest-Master.Medusa

01:13:55

passed SlapOS.Eggs.UnitTest-Master.Python2

00:12:32

passed SlapOS.Eggs.UnitTest-Master.Python3

00:14:05