yampa: CI job fails due to missing release file for postgresql repository

[ivanperez-keera]

The CI jobs are failing because apt-get is trying to update the package db and it's not finding some of the release files:

Err:8 http://apt.postgresql.org/pub/repos/apt bionic-pgdg Release
  404  Not Found [IP: 147.75.85.69 80]
[...]
Reading package lists... Done
E: The repository 'http://apt.postgresql.org/pub/repos/apt bionic-pgdg Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

This error is unrelated to Haskell. It's happening because bionic is no longer supported by postgresql.

Unfortunately, hvr didn't publish all ghc versions we test for in the successor jammy, so have one of three alternatives:

• Use a different installation tool.
• Drop support for some versions of GHC (or drop them from the tests).
• Remove postgresql.

I haven't had good experiences with ghcup, so I prefer, for now, to just remove postgresql since we don't need it.