Remove web-ui docs, except for info on upgrading to polymer

Also: Delete temporary file

Staged under:

https://webui-dot-dart-lang.appspot.com/

BUG= https://github.com/dart-lang/www.dartlang.org/issues/1193
R=sethladd@google.com

Review URL: https://chromiumcodereview.appspot.com/787303003