gh-35204: Remove directories leftover from improper shutdown so sage-cleaner won't kill random processes
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### 📚 Description
From #27769:
Under rare conditions, sage is killed at startup by sage-cleaner.
This requires:
Files left over in ~/.sage/temp/HOSTNAME/PID
A pid in ~/.sage/temp/HOSTNAME/PID/spawned_processes must exist, and
be started at init time. Its process group id will be 0
When sage-cleaner tries to clean up, it will do a "kill 0", which will
kill sage.
Files will only be left in ~/.sage/temp/HOSTNAME if sage is not shutdown
properly. Most likely because of a reboot.
Even if the situation doesn't result on a "kill 0", sage-cleaner would
still go on a killing spree for random processes which is awful.
The current PR removes everything under ~/.sage/temp/HOSTNAME/ at sage-
cleaner startup so this doesn't happen.
Since sage-cleaner won't run more than once at a time, this seems safe.
Fixes #27769
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### 📝 Checklist
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URL: https://github.com/sagemath/sage/pull/35204
Reported by: Gonzalo Tornaría
Reviewer(s):
