Cleanup: Remove unnecessary checks
Don't migrate buildversion on updatedb to PootleConfig

It won't work because the necessary tables are not yet present.
Also it is not necessary because this upgrade is already performed later
in the upgrade management command.
Recover ability to save buildversion using djblets

Since updatedb cannot rely on having the buildversions in PootleConfig
(see previous commit), we need to get back the legacy code to save the
buildversion using djblets.