Mark ModifiedNode as NotThreadSafe

The class is not intended to be threadsafe, so mmark it as such. Also
remove an invalid @GuardedBy annotation.

Change-Id: Ib2a5ee9710ff41447ff999d5056ae71d06b328a3
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "Mark ModifiedNode as NotThreadSafe"