Remove v8_interface.constructor_argument

Refactoring, no changes to generated bindings.

v8_interface.constructor_argument()
is redundant with
v8_methods.generate_argument()
(It's just a simplified version, as pointed out by Jens - thanks!)

There's one v. minor change required to merge this:
constructors have None as their idl_type, so check for that
in addition to 'void'.

R=haraken
BUG=345503

Review URL: https://codereview.chromium.org/274503002

git-svn-id: svn://svn.chromium.org/blink/trunk@173467 bbb929c8-8fbe-4397-9dbb-9b2b20218538