bindings: Removes unused code in IDL compiler (SpecialWrapFor, etc.)

SpecialWrapFor, is_document, is_audio_buffer, etc. are no longer used.
Removes them.

BUG=235436

Review URL: https://codereview.chromium.org/612343003

git-svn-id: svn://svn.chromium.org/blink/trunk@182907 bbb929c8-8fbe-4397-9dbb-9b2b20218538