Bug 1787794 - Part 7: Simplify paren-free arrow function parsing. r=arai

Instead of branching around `parenFreeArrow` in multiple places, handle the
paren-free arrow function case in a separate if-block. This also simplifies
the `Token::Modifier` handling, because we can always pass a constant.

Differential Revision: https://phabricator.services.mozilla.com/D155856