fields: Simplify HashField

fields: Simplify HashField

The HashField was written to be configurable. It supports customisable
hash types and provides fallbacks for a missing 'hashlib', which was
only introduced in Python 2.5. However, the customisable hash types
are not used anywhere (the actual hashing is hardcoded to use 'sha1')
and Python 2.7/3.3+ are the only versions of Python currently
supported. As a result, it is possible to remove much of the code
without losing any real functionality. Do this.

Signed-off-by: Stephen Finucane <stephen.finucane@intel.com>
Reviewed-by: Andy Doan <andy.doan@linaro.org>