(maint) Remove github macos tests

[rbrw]

These have been causing intermittent, and not so intermittent failures that appear to be due to extremely poor performance from the gihub runners, corroroborated by some online reports which also noted that they're expensive (per minute?).

Given that and the fact that no one's actively developing puppetdb from macos right now, remove the tests with the expectation that if/when we restore them, we'll restore them elsewhere.

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