drop legacy version handling from db schema

[sanderr]

Pre-iso4, we used a different db schema version scheme. inmanta.data.schema still has methods to interpret and convert this legacy scheme. I believe these methods are now redundant, since all versions < iso6 are no longer supported, therefore there are no more upgrade scenarios from the legacy version to the new ones (except perhaps in some of the older db schema migration test cases).

iso7 + iso8