[macOS] Eliminate macOS 10.14 availability check

[cbracken]

Now that the macOS embedders have all been updated to use a minimum macOS SDK of 10.14, eliminate the remaining @available checks dependent on that version.

This is one in a series of post-10.14 cleanup patches.

No tests are added since the code removed was a workaround for macOS 10.13. We now build with both a minimum SDK and deployment target of 10.14, and as such the workaround was already disabled at compile-time, making this change effectively a no-op.

Issue: flutter/flutter#114445

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[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie on the #hackers channel in Chat (don't just cc him here, he won't see it! He's on Discord!).

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[Hixie]

test-exempt: removing code

[dnfield]

Nice. I think this means we can remove an ANGLE/Vulkan work around as well...

[dnfield]

In //flutter/vulkan/procs/vulkan_interface.h we can remove lines 27-33

[dnfield]

And there's another macOS 10.14 check in FlutterRenderingBackend.mm

[cbracken]

Thanks! The first pass at this did a removal of the Vulcan undef but looked like it may have triggered a segfault in the impeller mac tests, so reverted that file. Will take another look shortly.

FlutterRenderingBackend.{h,mm} were removed in #37448.