Remove schema facility and most of formats

The usage of the securesystemslib schema/formats facility has been
little by little phased out and is now nowhere used anymore. Several
reasons to not use it are listed in #183.

This patch removes schema.py along with all securesystemslib schema
definitions in formats.py.

Additionally, it removes unrelated and unused helper functions in
formats.py.

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>