Remove outdated ObjC-related comments from the IDL

Remove outdated ObjC-related comments from the IDL since they no longer
make sense after the fork.

R=haraken
NOTRY=true

Review URL: https://chromiumcodereview.appspot.com/23001004

git-svn-id: svn://svn.chromium.org/blink/trunk@156215 bbb929c8-8fbe-4397-9dbb-9b2b20218538