Deprecate PropertyCacher.py

The pyphosphor package provides equivalent function
so use that and drop the duplicated code.

Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/bin/system_manager.py b/bin/system_manager.py
index 633fd25..0cfcd0e 100755
--- a/bin/system_manager.py
+++ b/bin/system_manager.py
@@ -8,7 +8,7 @@
 import dbus.mainloop.glib
 import os
 import time
-import PropertyCacher
+from obmc.dbuslib.propertycacher import PropertyCacher
 from obmc.dbuslib.bindings import DbusProperties, DbusObjectManager, get_dbus
 import obmc.enums