[keyserver] Remove links from docker-compose.yml

Summary:
In Docker Compose 2, links declared what other services were linked from a service. It was deprecated in favor of networks.

In Docker Compose 3, links declares alias hostnames by which other services can access a service.

I accidentally copied links from a StackOverflow snippet. @jimpo's comment here made me investigate and realize this was a mistake.

Test Plan: docker-compose up --build

Reviewers: palys-swm, jimpo

Reviewed By: palys-swm

Subscribers: Adrian, atul, karol-bisztyga, yayabosh, jimpo

Differential Revision: https://phabricator.ashoat.com/D3869