Remove unused AudioParams in PannerNode.

m_distanceGain and m_coneGain are not used internally in panner node.
Remove useless valuables.

BUG=none

Review URL: https://codereview.chromium.org/231033002

git-svn-id: svn://svn.chromium.org/blink/trunk@171214 bbb929c8-8fbe-4397-9dbb-9b2b20218538