[Yoga] Delete YOGA_TREE_CONTIGOUS gating and permanently enable. #trivial

[appleguy]

NOTE: Yoga support is still highly experimental and is not
planned to become a supported / documented mode of the framework.
We recommend that most apps use ASStackLayoutSpec and the other
specs: http://texturegroup.org/docs/layout2-layoutspec-types.html

This reduces complexity of the Yoga integration points, and is
a first step towards further minimization and formalization of
the Yoga footprint in the framework.

[ghost]

🚫 CI failed with log

[appleguy]

@garrettmoon is there a comment I can post that triggers a build re-run? This appears to be a spurious failure:

  testThatMultipleBatchFetchesDontHappenUnnecessarily, failed - Too many batch fetches!
  /Users/Shared/buildkite/builds/iosf-garrett-VMrmmvuqKsO4.dyn.pinadmin.com-1/pinterest/texture/Tests/ASCollectionViewTests.mm:891

if (batchFetchCount > 1) {
  XCTFail(@"Too many batch fetches!");
  return;

   Executed 452 tests, with 1 failure (0 unexpected) in 23.750 (24.213) seconds
Failing tests:
  -[ASCollectionViewTests testThatMultipleBatchFetchesDontHappenUnnecessarily]
** TEST FAILED **

[ghost]

	1 Warning
⚠️	Any source code changes should have an entry in CHANGELOG.md or have #trivial in their title.

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