oeqa/selftest/case: Use bb.utils.remove() instead of shutil.remove()
This avoids problems where shutil.remove will error with:
File "/usr/lib/python3.5/shutil.py", line 436, in _rmtree_safe_fd
os.unlink(name, dir_fd=topfd)
FileNotFoundError: [Errno 2] No such file or directory: 'S.gpg-agent.extra'
when there are races over file deletion (gpg agent may be slow to exit).
We already worked around speed and race issues in bb.utils.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
