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.