chore: remove per-workflow Slack notifier

[Dominik Grothaus (dgrothaus-sw)]

Nightly failure notifications are now handled centrally by disaster-central.
The per-workflow notify job is no longer needed.

[Codecov Comments Bot (codecov-commenter)]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.72%. Comparing base (67b8a1e) to head (67d1cbc).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master      #65   +/-   ##
=========================================
  Coverage     81.72%   81.72%           
  Complexity      135      135           
=========================================
  Files            21       21           
  Lines           662      662           
=========================================
  Hits            541      541           
  Misses          121      121           

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