Fix 2008 Maryland general election validation

It reflected the wrong election ID due to the datasource not handling
the 2008 special election.

Addresses https://github.com/openelections/core/issues/36,
https://github.com/openelections/core/issues/107
Remove references to filenames.json

The use of this file has been deprecated in favor of other
Datasource methods.

Removed reference to filenames.json in OH, MD and ID datasources.

Removed the ``clear_filenames()``, ``filename_mappings()`` and
``update_mappings()`` methods from ``base.datasource.Datasource``.
These methods referenced ``filenames.json``.  As far as I could tell
with a quick grep, these weren't being called from anywhere.

Closes https://github.com/openelections/core/issues/112