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/dhcp_config_unittest.cc b/dhcp_config_unittest.cc
index 45c62fc..d6d3929 100644
--- a/dhcp_config_unittest.cc
+++ b/dhcp_config_unittest.cc
@@ -56,7 +56,7 @@
    public:
     explicit TestProxyFactory(DHCPConfigTest *test) : test_(test) {}
 
-    virtual DHCPProxyInterface *CreateDHCPProxy(const string &service) {
+    virtual DHCPProxyInterface *CreateDHCPProxy(const string &/*service*/) {
       return test_->proxy_.release();
     }