refactor(externs): move Closure Externs back to Closure code repository

While Closure Compiler generally recommends to maintain the externs for
projects together with their source, this has not worked well for
AngularJS:
- Changes to externs must be tested; they can break clients. AngularJS
  has no testing infrastructure for this.
- Changes mostly come from users inside of Google and are much more
  easily submitted together with the code using them within Google's
  repository.

This change deletes the externs here and adds a README.closure.md to
document the change. They will be added back to Closure Compiler in a
separate submit.

Closes #5906

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