Cleanup constants in IDL compiler + remove conditional constants

Followup to:
Add constants and primitive type readonly attributes to Python IDL compiler
https://codereview.chromium.org/23068032/

This changes formatting and removes support for [Conditional] constants,
as there are none in actual IDLs and Conditional is deprecated.
This makes the code significantly simpler.
Also some minor code factoring changes.

Review URL: https://chromiumcodereview.appspot.com/23455010

git-svn-id: svn://svn.chromium.org/blink/trunk@156778 bbb929c8-8fbe-4397-9dbb-9b2b20218538