shill: enable (don't disable) -Wunused

having fixed an unused parameter in the dbus-c++ headers, those
headers no longer have unused parameters. so, if we fix up unused
parameters in shill code, we can compile with -Wunused. (this is
implied by -Wall, as long as we don't override with -Wno-unused.)

BUG=chromium-os:20779
TEST=unittests

- comment out unused parameters in function definitions
- remove some unused local variables
- also: fix an old-style cast

Change-Id: Ie654a7b34d622cecb0f26bd47c17813fe8423001
Reviewed-on: http://gerrit.chromium.org/gerrit/8365
Tested-by: mukesh agrawal <quiche@chromium.org>
Reviewed-by: Paul Stewart <pstew@chromium.org>
diff --git a/wifi_service.cc b/wifi_service.cc
index cba5a2a..881bf1d 100644
--- a/wifi_service.cc
+++ b/wifi_service.cc
@@ -64,7 +64,7 @@
   LOG(INFO) << __func__;
 }
 
-void WiFiService::Connect(Error *error) {
+void WiFiService::Connect(Error */*error*/) {
   LOG(INFO) << __func__;
 
   // NB(quiche) defer handling, since dbus-c++ does not permit us to