shill: improve logging, and comment out unused args

1) Add some more context for a few logging calls.
2) Comment out some unused args, to make it obvious that they're
   unused.

BUG=None
TEST=unit tests

Change-Id: I0a71a8d2311a17859f96793a88f950c3636cd91e
Reviewed-on: https://gerrit.chromium.org/gerrit/61601
Commit-Queue: mukesh agrawal <quiche@chromium.org>
Reviewed-by: mukesh agrawal <quiche@chromium.org>
Tested-by: mukesh agrawal <quiche@chromium.org>
diff --git a/cellular.cc b/cellular.cc
index 678f75b..7f29c19 100644
--- a/cellular.cc
+++ b/cellular.cc
@@ -474,6 +474,7 @@
 }
 
 void Cellular::DestroyService() {
+  SLOG(Cellular, 2) << __func__;
   DropConnection();
   if (service_) {
     LOG(INFO) << "Deregistering cellular service " << service_->unique_name()