mgmt: add FaceStatus::unsetExpirationPeriod()

For consistency with Route.

Change-Id: Ieac12ebd7fc7270c03d331c48747bd913f79fef9
Refs: #3903
diff --git a/src/mgmt/nfd/face-status.hpp b/src/mgmt/nfd/face-status.hpp
index bde34b8..32c2607 100644
--- a/src/mgmt/nfd/face-status.hpp
+++ b/src/mgmt/nfd/face-status.hpp
@@ -74,6 +74,9 @@
   FaceStatus&
   setExpirationPeriod(time::milliseconds expirationPeriod);
 
+  FaceStatus&
+  unsetExpirationPeriod();
+
   uint64_t
   getNInInterests() const
   {