Remove unused `postgis` gem

[rambip]

The postgis module is not currently used. It was added to use Decidim-geo, but this module is not used here.

If it is still used, it should appear in the development install instructions see #895

[rambip]

I had a lot of difficulty installing postgis, here is a summary as a reminder.

I installed the default stable Postgres version with brew install postgresql. Unfortunately, this version of Postgres is @14.
Then, bundle install linked the pg gem to the local install of Postgres in my system.

Then, brew install postgis failed, with a similar error as the one described in this issue. This was probably because of an outdated version of macOS.

To get a functioning system, here is what I did:

• update entirely my Mac
• uninstall postgis and postgres (brew rm postgis; brew rm postgresql). Also check you don't have other versions of postgresql with `
• reinstall Postgres with a newer version (brew install postgresql@17)
• reinstall postgis (brew install postgis)
• restart postgresql (brew services start postgresql@17)
• remove the pg gem and reinstall it, so it points to the right version (bundle exec gm uninstall pg -aIx; bundle install)

I also had a weird "UTC" error like in this post, so I rebooted my Mac.

The important thing is to do these steps in the right order, so that postgis links to the right version of postgresql, and the gem points to the right version of postgresql AND postgis.