Removes TUF-specific formats from formats.py
Removes TUF-specific formats from formats.py

Largely resolves Issue #161, especially given the merge of
PR #162.

See GitHub:
https://github.com/secure-systems-lab/securesystemslib/issues/161
https://github.com/secure-systems-lab/securesystemslib/pull/162

Signed-off-by: Sebastien Awwad <3db59c6db3a806262760013e1ae477b12a72032f@gmail.com>

secure-systems-lab/securesystemslib,secure-systems-lab/securesystemslib