erp5_core: remove unused code

renderJS's mutex must be used instead
6 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-Master

00:46:42

passed ERP5.PerformanceTest-Master

00:23:31

failed ERP5.UnitTest-Master

01:27:48

failed ERP5.UnitTest-Master.Medusa

01:12:59

passed SlapOS.Eggs.UnitTest-Master.Python2

00:16:14

passed SlapOS.Eggs.UnitTest-Master.Python3

00:26:35