Remove installation of `postgresql-contrib` in "System packages" section in install.md

[alxhu-dev]

The postgresql-contrib meta package has been removed with the release of postgresql-common v271 in February 2025 (see here) because it's served now by postgresql.

Some Linux distros may provide their own postgresql-contrib packages, but the Mastodon documentation states that you should use the PGDG APT repository so we should stick to available & supported packages and not mix diffrent repositories for the same component.