meta: Delete stale.yml workflow

[szokeasaurusrex]

We have deleted this workflow in other repos (e.g. getsentry/sentry-python#2245), and we should probably delete it here as well, by the same logic.

[szokeasaurusrex]

Before merging this PR, we need to make sure that the new stalebot logic also applies to the sentry-cli repo. I found that the list of repos is defined here as a concatenation of SENTRY_MONOREPOS and SENTRY_SDK_REPOS, but since I was unable to find where these constants are defined, I am unsure whether sentry-cli is included. If it is not included, maybe it should be.

@chadwhitacre or @hubertdeng123, can one of you confirm whether sentry-cli is being monitored by the new stalebot logic?

[hubertdeng123]

Yep, sentry-cli is being monitored by stalebot in eng-pipes: https://github.com/getsentry/eng-pipes/blob/950e5365bba331bee1cbdf063ac2ef78ecd11856/github-orgs.yml#L55

This should be safe to do