Remove part of a column that no longer applies

Now there are no dynamically created parties that just pop into
existence.
Remove a now-unused variable
Make sure that 'Independent' appears as an option in the party lists

... as well as other pseudo parties (e.g. "Speaker seeking re-election")
and a few parties mentioned in the 2010 YourNextMP data that have no
matches in the Electoral Commission database.

Fixes #70