Removing leftover db-proxy bits

[vivian-rook]

A few variables for db-proxy were left behind. One of which,
MYSQL_SERVICE_HOST from the proxy service, was causing a
failure, as it was no longer set.

Bug: T310363

[vivian-rook]

@chicocvenancio talked a little more about what was happening in https://phabricator.wikimedia.org/T310363. Looks like some bits from db-proxy were cached and failed after I messed around with my dev env enough. I assume eventually the same will happen in prod. I believe everything I'm removing is old db-proxy stuff, but was hoping for you to take a look before I send it out to prod.

Thanks!

[chicocvenancio]

It all looks related, but I'm trying to remember what is the current way to access db replicas and if it depends on any of these vars.

[vivian-rook]

I did see that variable and the others in the git history in db-proxy/images and the db-proxy template. I mostly test replica connections with the script in:
https://public.paws.wmcloud.org/User:SRodlund_(WMF)/%2A2021%20UPDATED%2A%20Replica%20Helper%20%26%20Database%20connections%20with%20PAWS.ipynb#connections
which uses .my.cnf for login info, I've yet to establish where the .my.cnf file comes from though.