IDL compiler: fix Python sync in r163467

In:
Remove UnusedParam.h
https://codereview.chromium.org/34273002/
...there was one excess ALLOW_UNUSED in the Python CG that had been
removed in the Perl CG, so they were inconsistent.

This fixes it, making the Python consistent with Perl.

Specifically, the excess ALLOW_UNUSED was removed from Perl (but not all
Python) in:
Patch Set 5 : Remove unnecessary ALLOW_UNUSED from the binding generator
https://codereview.chromium.org/34273002/#ps550001

Compare:
https://codereview.chromium.org/34273002/diff2/530001:550001/Source/bindings/scripts/code_generator_v8.pm
https://codereview.chromium.org/34273002/diff2/490001:550001/Source/bindings/templates/interface.cpp

TBR=haraken
NOTRY=true

Review URL: https://codereview.chromium.org/110993004

git-svn-id: svn://svn.chromium.org/blink/trunk@163481 bbb929c8-8fbe-4397-9dbb-9b2b20218538