security+util: remove overloaded operator!()

Identity, Key, and EventId already provide an operator bool(),
which covers all valid use cases.

Change-Id: I57b11db6997e6f537873f41c6ca1c22140b894c3
diff --git a/src/security/pib/key.cpp b/src/security/pib/key.cpp
index 7396a5a..efb7df5 100644
--- a/src/security/pib/key.cpp
+++ b/src/security/pib/key.cpp
@@ -102,13 +102,7 @@
 
 Key::operator bool() const
 {
-  return !(this->operator!());
-}
-
-bool
-Key::operator!() const
-{
-  return m_impl.expired();
+  return !m_impl.expired();
 }
 
 shared_ptr<detail::KeyImpl>