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.hpp b/src/security/pib/key.hpp
index fbbed24..b691960 100644
--- a/src/security/pib/key.hpp
+++ b/src/security/pib/key.hpp
@@ -77,7 +77,7 @@
   explicit
   Key(weak_ptr<detail::KeyImpl> impl);
 
-  /*
+  /**
    * @brief Get key name.
    */
   const Name&
@@ -128,12 +128,6 @@
   explicit
   operator bool() const;
 
-  /**
-   * @brief Check if the Key instance is invalid.
-   */
-  bool
-  operator!() const;
-
 NDN_CXX_PUBLIC_WITH_TESTS_ELSE_PRIVATE: // write operations should be private
   /**
    * @brief Add @p certificate.