IDL compiler: remove blank line for void methods

Void methods have a gratuitous blank line.
This removes it; it makes the Perl CG a bit more complex (one line),
but will make the Python simpler, b/c we will already check for void
in the template.

BUG=239771

Review URL: https://codereview.chromium.org/48003002

git-svn-id: svn://svn.chromium.org/blink/trunk@160696 bbb929c8-8fbe-4397-9dbb-9b2b20218538