ci: remove duplicate guardrails [skip deploy]

[scottybarr]

Why

We recently rolled out this new Guardrails workflow to every repository, but
I've discovered we don't actually need it! 🙈

I learned that GitHub rulesets can require a specific, shared workflow to pass
for every repository instead! So, I have created an org ruleset that requires
the
guardrails workflow
in github-workflows to pass before merging to the default branch. This applies
to all repositories, but it is currently not a blocker for merging.

The problem we have now is that there are two guardrails checks running on
every PR (the one from the ruleset and the one from the workflow in each
repo).

Maintaining a single ruleset is easier than maintaining workflow files in our
~400 active repositories. So let's keep it simple!

https://wealthsimple.atlassian.net/browse/BEPLAT-1468

What Changed

• Delete the guardrails.yml GitHub workflow file

[!WARNING] This PR was opened automatically Please review it carefully for
changes! Reach out to
#backend-platform-on-call
on Slack if you have any questions

[cchawn]

I actually want to keep this for public repos, since we've excluded them from the "Enforce Guardrails" ruleset.