[docs] Remove outdated section about automatic tree closures

No-Try: true
Docs-Preview: https://skia.org/?cl=147965
Bug: skia:
Change-Id: Ia5753e125563f97ada724eae3fad3b497ea20403
Reviewed-on: https://skia-review.googlesource.com/147965
Reviewed-by: Ben Wagner <benjaminwagner@google.com>
Commit-Queue: Eric Boren <borenet@google.com>
1 file changed