Cleanup of unused functions

The cleanup also found out that custom translation for std-derived
exceptions test wasn't running properly, and fixed that.
We cannot enable the warning globally, because the tests contain
some functions that are unused by design -- e.g. when checking
stringification priority of StringMaker vs range fallback and so
on.

tests/SelfTest/UsageTests/Exception.tests.cpp

@@ -39,7 +39,7 @@ namespace {
         explicit CustomException(const std::string& msg)
             : m_msg(msg) {}
 
-        std::string getMessage() const {
+        std::string const& getMessage() const {
             return m_msg;
         }
 
@@ -51,9 +51,12 @@ namespace {
     public:
         explicit CustomStdException(const std::string& msg)
             : m_msg(msg) {}
-        ~CustomStdException() noexcept override {}
+        ~CustomStdException() noexcept override = default;
 
-        std::string getMessage() const {
+        CustomStdException( CustomStdException const& ) = default;
+        CustomStdException& operator=( CustomStdException const& ) = default;
+
+        std::string const& getMessage() const {
             return m_msg;
         }
 
@@ -137,7 +140,7 @@ TEST_CASE("Non-std exceptions can be translated", "[.][failing][!throws]" ) {
 }
 
 TEST_CASE("Custom std-exceptions can be custom translated", "[.][failing][!throws]" ) {
-    throw CustomException( "custom std exception" );
+    throw CustomStdException( "custom std exception" );
 }
 
 TEST_CASE( "Custom exceptions can be translated when testing for nothrow", "[.][failing][!throws]" ) {