Account preferences clean-up

There were a number of preferences that were not being removed
from the preferences DB when an account was deleted, so they
would remain there forever.

There were a few attempts to remove preference keys from the DB where
the keys were obsolete and not in use for some time.

Certain obsolete preferences were not modified:
mUuid + ".name"
mUuid + ".email"
mUuid + ".signature"
mUuid + ".signatureUse"
These were in use before implementing multiple identities, and are still used
as a fallback for old accounts without multiple identities configured.