Restore gitaly default log level to 'info'

In !2642 (merged) we added code that overrides the default Gitaly log level. I suspect that was by accident.

If it was intentional then it still doesn't make sense. If we think the Gitaly defaults are bad for users we should fix them in gitlab-org/gitaly, not override them in Omnibus.

BTW, log level 'info' had a lot of spammy messages in it from the grpc-go library. That got fixed in gitaly!765 (merged) already.