src: make PercentDecode return void

It only returns 0, nor is it likely to have any error conditions in the
future.

PR-URL: https://github.com/nodejs/node/pull/11922


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>