build: Drop codeql.

[feanil]

We were using this repo and a few others to test this but have not
really actioned on it and have not seen a lot of value from this. The
latest version doesn't work out of the box with the repo either. So
let's drop it for now and we can come back to it when we're ready to
spend more time on it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 55.35%. Comparing base (db2760e) to head (93dac8b).

Additional details and impacted files

@@ Coverage Diff @@## master #404 +/- ##
=======================================
Coverage 55.35% 55.35% =======================================
Files 3 3 Lines 56 56 =======================================
Hits 31 31 Misses 25 25 

Flag	Coverage Δ
unittests	55.35% <ø> (ø)

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