ci: remove changelog-preview.yml (#112408)

this currently runs for 4 minutes everytime someone commits to a PR or
applies labels etc. etc.

and it's not clear to me if it's doing anything useful

was removed in #108055 then
reintroduced in #108061 where pr
comments were disabled for changelog summary - fine. but it's still
taking 4m to generate it, which gets printed to pages like
https://github.com/getsentry/sentry/actions/runs/24102901929 which i
feel nobody ever reads

Author: joshuarli