Retire Delight2 migration code to speed up tests.

We're waiting 10 minutes for tests to run, and half of that time is spent in
depreacted code related to migration of Delight2 dictionary files.
LatinIME will never migrate another Delight2 dictionary file again, so we can
delete this code.

Change-Id: I05c7d8429e8d9a26139456763c77997340fea8c2