Revert increased inline threshold (iff Clang) to exported Ceres target.

- Increasing the inline threshold results in very variable performance
  improvements, and could potentially confuse users if they are trying
  to set the inline threshold themselves.
- As such, we no longer export our inline threshold configuration for
  Clang, but instead document how to change it in the FAQs.

Change-Id: I88e2e0001e4586ba2718535845ed1e4b1a5b72bc