Remove references to TUF

[vladimir-v-diaz]

Description of issue or feature request:

securesystemslib was not always a library. Many of its modules and functions previously lived in the TUF repository. Unfortunately, there are still references to TUF in some of the securesystemslib comments.

Current behavior:
There are references to TUF in comments and there exist schemata that are only relevant to TUF. For example:

securesystemslib/securesystemslib/formats.py

Lines 18 to 24 in d111604

	A central location for all format-related checking of TUF objects.
	Note: 'formats.py' depends heavily on 'schema.py', so the 'schema.py'
	module should be read and understood before tackling this module.
	'formats.py' can be broken down into three sections. (1) Schemas and object
	matching. (2) Classes that represent Role Metadata and help produce
	correctly formatted files. (3) Functions that help produce or verify TUF

securesystemslib/securesystemslib/formats.py

Lines 326 to 334 in d111604

	# Version information specified in "snapshot.json" for each role available on
	# the TUF repository. The 'FILEINFO_SCHEMA' object was previously listed in
	# the snapshot role, but was switched to this object format to reduce the
	# amount of metadata that needs to be downloaded. Listing version numbers in
	# "snapshot.json" also prevents rollback attacks for roles that clients have
	# not downloaded.
	VERSIONINFO_SCHEMA = SCHEMA.Object(
	object_name = 'VERSIONINFO_SCHEMA',
	version = METADATAVERSION_SCHEMA)

Expected behavior:
TUF references in securesystemslib should be removed and TUF-related schemata relocated to the TUF repository.

[JustinCappos]

This is a low priority, cleanup item...

[lukpueh]

Fixed with #165.