medusa: remove postgresql variant

[dgilman]

autoconf does not support pg_config discovery of postgresql and its paths, a requirement for installation with modern versions of postgresql.

Description

Type(s)

• bugfix
• enhancement
• security fix

Tested on

macOS 14.5 23F79 x86_64
Command Line Tools 15.3.0.0.1.1708646388

Verification

Have you

• followed our Commit Message Guidelines?
• squashed and minimized your commits?
• checked that there aren't other open pull requests for the same change?
• referenced existing tickets on Trac with full URL in commit message?
• checked your Portfile with port lint --nitpick?
• tried existing tests with sudo port test?
• tried a full install with sudo port -vst install?
• tested basic functionality of all binary files?
• checked that the Portfile's most important variants haven't been broken?

[macportsbot]

Notifying maintainers:
@Gminfly for port medusa.