Remove type of panningModelType "soundfield".

One of panningModelType "soundfiled" had been not implemented until now.
In addition, the last W3C Spec about webaudio, has been deleted about this feature.
http://www.w3.org/TR/webaudio/

The code about "soundfiled" seems to be removed in blink.

BUG=346191

Review URL: https://codereview.chromium.org/176873003

git-svn-id: svn://svn.chromium.org/blink/trunk@167750 bbb929c8-8fbe-4397-9dbb-9b2b20218538