exception: Remove unused sdbus_impl declaration

Change 9ff25f7365d4d590fcb5349328a46a8cd2093f34 exposed the sdbus_impl
globally from the sdbus.hpp file. Remove this leftover declaration
since it is no longer needed.

Tested:
    Built and run through the unit test suite.

Change-Id: I39484959978b07dd27aa3b3f3c0f046c5aa9b515
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/sdbusplus/exception.cpp b/sdbusplus/exception.cpp
index f13ba86..2e3b88a 100644
--- a/sdbusplus/exception.cpp
+++ b/sdbusplus/exception.cpp
@@ -1,11 +1,8 @@
 #include <sdbusplus/exception.hpp>
-#include <sdbusplus/sdbus.hpp>
 #include <stdexcept>
 #include <system_error>
 #include <utility>
 
-extern sdbusplus::SdBusImpl sdbus_impl;
-
 namespace sdbusplus
 {
 namespace exception
diff --git a/sdbusplus/exception.hpp b/sdbusplus/exception.hpp
index 6cfc0f4..9f4caa5 100644
--- a/sdbusplus/exception.hpp
+++ b/sdbusplus/exception.hpp
@@ -6,8 +6,6 @@
 #include <system_error>
 #include <systemd/sd-bus.h>
 
-extern sdbusplus::SdBusImpl sdbus_impl;
-
 namespace sdbusplus
 {