Modify Gitlab::AuthorizedKeys#rm_key and #list_key_ids to
handle cases when an `Errno::ENOENT` is raised.
Also added test cases in spec to ensure each method will
still work when authorized_keys doesn't exist.
In this commit, some methods that aren't being used
are removed from `Gitlab::Shell`. They are the ff:
- `#remove_keys_not_found_in_db`
- `#batch_read_key_ids`
- `#list_key_ids`
The corresponding methods in `Gitlab::Keys` have been
removed as well.
