Fix pending call unref convention for dbus_send() helper functions

Legacy way of dealing with asynchronous dbus method calls in mce was to
a) leave one extra ref to pending call when issuing the method call
b) call dbus_pending_call_unref() from handler callback
Which is problematic since (b) does not happen if the pending call is
canceled.

Now the extra reference is cleared after it is no longer needed and
calls to dbus_pending_call_unref() from all related handler callbacks
are removed.