oeqa/selftest/case: Use bb.utils.remove() instead of shutil.remove()

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.

(From OE-Core rev: 00a8fd5b93a5c19ce0b7498e2bc653ce8ad58aaf)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>