Remove Heroku-specific code and references

[mathjazz]

Fix #3843.

[flodolo]

I confess I'm not a fan of leaving so much code specific for Heroku around.

Can we have a lower priority issue with the list of things that are only needed by Heroku, and get rid of them at some point.

[flodolo]

As for the PR title, this is not removing Heroku code, it's only removing some references as a supported platform.

According to the issue, we should also remove https://github.com/mozilla/pontoon/blob/main/package.json#L22?

[mathjazz]

I confess I'm not a fan of leaving so much code specific for Heroku around.

We could get rid of these:
https://github.com/mozilla/pontoon/blob/main/app.json
https://github.com/mozilla/pontoon/blob/main/pontoon/base/management/commands/heroku_deploy_setup.py

The rest (it's really just 2 files in pontoon/bin and a line in package.json at that point) I'd keep, because we most likely have deployments that rely on them.

[mathjazz]

According to the issue, we should also remove https://github.com/mozilla/pontoon/blob/main/package.json#L22?

Nah, that one should remain, otherwise npm run build:prod will run twice on Heroku deployments.

[mathjazz]

As for the PR title, this is not removing Heroku code, it's only removing some references as a supported platform.

It's removing Heroku-specific code from the settings file.