erp5testnode: fix clean up of old instance root folder under Python 3

This fixes commit ea4cb55b.

Our 'rmtree' helper has never been supposed to be called on a
non-existing folder: the ENOENT handling is an unrelated
implementation detail for Python 2.

Also drop erp5.util's rmtree and use the one from slapos.core,
which is a [testnode] dependency.
12 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-Master

01:20:09

passed ERP5.PerformanceTest-Master

00:24:18

failed ERP5.UnitTest-Master

01:48:13

failed ERP5.UnitTest-Master.Medusa

04:11:26

passed SlapOS.Eggs.UnitTest-Master.Python2

00:15:10

passed SlapOS.Eggs.UnitTest-Master.Python3

00:08:53

failed ERP5.UnitTest-Master

02:32:33

passed SlapOS.Eggs.UnitTest-Master.Python2

00:19:20

passed SlapOS.Eggs.UnitTest-Master.Python2

00:06:45

passed SlapOS.Eggs.UnitTest-Master.Python3

00:12:14

passed SlapOS.Eggs.UnitTest-Master.Python3

00:18:19

passed SlapOS.Eggs.UnitTest-Master.Python3