IDL compiler: remove nop return from end of getters and setters

Getters and setters are void functions.
Currently they end with a useless return; this removes them.

BUG=239771

Review URL: https://codereview.chromium.org/25675003

git-svn-id: svn://svn.chromium.org/blink/trunk@158695 bbb929c8-8fbe-4397-9dbb-9b2b20218538