Remove chainrule and test for SqMahalanobis

[simsurace]

Just some cleanup work to remove remnants of the Mahalanobis kernel which has been absent for a while.

• They are no longer needed
• The chainrule is committing type piracy towards Distances.jl (would there be value to move the rule in the extension there?)
• The test is triggering some warnings that are distracting

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (ec19a94) 90.43% compared to head (a8d07c5) 90.34%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #539      +/-   ##
==========================================
- Coverage   90.43%   90.34%   -0.09%     
==========================================
  Files          52       52              
  Lines        1390     1378      -12     
==========================================
- Hits         1257     1245      -12     
  Misses        133      133              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[willtebbutt]

Approved subject to patch bump, so that we can tag a release

[simsurace]

Hi @willtebbutt, the version was already bumped, but I haven't released it just yet (was hoping to figure out the failures on master first). Do we merge this anyway and release it with the other fixes once they are done?

[willtebbutt]

Ah okay, I hadn't realised we were in this situation. As a rule, we never leave an unreleased version of the package on master -- if a PR (with patch bump) is good enough for a release.

Could you please release the current version of master, then bump + release this? I grant you that it's annoying that there are failures on master, but unless you think that they're related to a PR that has recently been merged, I don't feel that they ought to disrupt the release cycle.

[simsurace]

No the remaining failures are likely due to relying on some internals in Julia, and were introduced by the Julia 1.8 -> 1.9 update. I will do as you suggest.