Remove no-longer-needed imports in util.py
Remove no-longer-needed imports in util.py

sys and fnmatch are no longer needed due to the removal of unused
functions in prior commits.

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

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