Do not try to remove sha1 cache and changelog files that do not exist

Traceback (most recent call last):
  File "bin/remove-obsolete-new-packages", line 178, in <module>
    remove(sha1_cache_path, 'file from sha1 cache')
  File "bin/remove-obsolete-new-packages", line 122, in remove
    os.rmdir(os.path.dirname(path))
  File "/usr/lib/python3.5/posixpath.py", line 148, in dirname
    i = p.rfind(sep) + 1
AttributeError: 'NoneType' object has no attribute 'rfind'

Traceback (most recent call last):
  File "bin/remove-obsolete-new-packages", line 181, in <module>
    remove(changelog_path, 'changelog file')
  File "bin/remove-obsolete-new-packages", line 122, in remove
    os.rmdir(os.path.dirname(path))
  File "/usr/lib/python3.5/posixpath.py", line 148, in dirname
    i = p.rfind(sep) + 1
AttributeError: 'NoneType' object has no attribute 'rfind'