Remove useless function and typs in the V8Callback

CallbackAllowedValueFlags type and createFunctionOnlyCallback in the
"V8Callback" class are not used.
They have been only used for "V8GeolocationCustom.cpp", but that file was
removed already.

BUG=385426

Review URL: https://codereview.chromium.org/344423002

git-svn-id: svn://svn.chromium.org/blink/trunk@176726 bbb929c8-8fbe-4397-9dbb-9b2b20218538