CI: Don't split slow vs non slow jobs for GHA posix
Since GHA has a 6 hour job limit, and it appears that slow jobs take ~25 mins and non-slow jobs take ~35 minutes, it shouldn't be necessary to split these anymore.


Helps reduce the number of builds so more can run concurrently.
this needed? Yup this is an orthogonal cleanup. All good here. Failure unrelated.  @mroeschke needs a rebase @mroeschke one more rebase if you can and https://github.com/pandas-dev/pandas/runs/4932326914?check_suite_focus=true failed (but looks suprios) @meeseeksdev backport 1.4.x Something went wrong ... Please have a look at my logs.