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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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 ```

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[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.