Don't process dup unwanted() when unregistering NetworkAgent

In cases where the NetworkAgent is unregistered (eg. Safe mode), but the
VCN is not shut down, the NetworkAgent.unwanted() call should not
trigger a teardown.

Test: atest FrameworksVcnTests
Change-Id: Icf32cb464bce2aae2846448d5a6a53a97f558398