Remove mostly unused util.py

The util.py module provides a mix of minimal functions related to local files
and directories. These functions are mostly unused, exceptions are
(according to sourcegraph.com):

- persist_temp_file in python-tuf (cc @jku)
- get_file_hashes in scai-demos (cc @marcelamelara, @adityasaky)
- get_file_details, load_json_file in taf (cc @renatav, @adityasaky)

It seems feasible to reimplement the desired functionality downstream.

The benefits of removing util.py are:

- minimizes the API surface
- removes one dependent of the about to be removed schema.py (secure-systems-lab#183)
- makes unittest_toolbox obsolete

Signed-off-by: Lukas Puehringer <[email protected]>

mypy.ini

@@ -1,7 +1,6 @@
 [mypy]
 warn_unused_configs = True
 files =
-    securesystemslib/util.py,
     securesystemslib/signer/*.py,
     securesystemslib/storage.py,
     securesystemslib/gpg/constants.py