Remove unused functions in PositionOptions

setEnableHighAccuracy, setTimeout and setMaximumAge are only used in
"V8GeolocationCustom.cpp", but that file has been removed on the r176719.
Therefore those functinos have to be removed also.

BUG=385426

Review URL: https://codereview.chromium.org/348253003

git-svn-id: svn://svn.chromium.org/blink/trunk@176744 bbb929c8-8fbe-4397-9dbb-9b2b20218538