Remove deferred loading page; tweak async & deferred loading text

I also tweaked some code, which will be in a separate pull request.

Staged at:
https://spec-dot-dart-lang.appspot.com/docs/spec/
https://spec-dot-dart-lang.appspot.com/docs/spec/deferred-loading.html
(redirects to https://spec-dot-dart-lang.appspot.com/docs/dart-up-and-running/ch02.html#deferred-loading)
https://spec-dot-dart-lang.appspot.com/docs/dart-up-and-running/ch02.html#asynchrony

R=sethladd@google.com

Review URL: https://chromiumcodereview.appspot.com/789393002