GD-Cert: Fix --clean option when RAM disk is used

* When RAM disk is used, $CERT_TEST_VENV is a symbolic link, rm -rf
  this directory will only remove the symbolic link instead of cleaning
  up its content

Bug: 159255653
Fixes: 159255653
Tag: #gd-refactor
Test: gd/cert/run --clean
Change-Id: I713a1a0df737aacec4e51bfa58e8ca6f8a52c2b8