remove unavailable mac 12 tester.

[MFraters]

The mac-12 tester is not available anymore, so just remove it.

[github-actions]

Benchmark	Main	Feature	Difference (99.9% CI)
Slab interpolation simple none	1.116 ± 0.011 (s=400)	1.115 ± 0.009 (s=409)	-0.3% .. +0.1%
Slab interpolation curved simple none	1.120 ± 0.010 (s=425)	1.122 ± 0.010 (s=380)	-0.0% .. +0.4%
Spherical slab interpolation simple none	1.098 ± 0.010 (s=422)	1.097 ± 0.011 (s=401)	-0.3% .. +0.1%
Slab interpolation simple curved CMS	1.166 ± 0.011 (s=383)	1.165 ± 0.010 (s=392)	-0.3% .. +0.2%
Spherical slab interpolation simple CMS	1.446 ± 0.013 (s=298)	1.445 ± 0.012 (s=327)	-0.3% .. +0.1%
Spherical fault interpolation simple none	1.101 ± 0.009 (s=410)	1.101 ± 0.010 (s=410)	-0.2% .. +0.2%
Cartesian min max surface	2.540 ± 0.016 (s=189)	2.540 ± 0.018 (s=168)	-0.2% .. +0.3%
Spherical min max surface	7.176 ± 0.083 (s=61)	7.169 ± 0.078 (s=67)	-0.8% .. +0.6%