cmTarget: Remove unneeded constructors.

The compiler generated ones are fine here.

Source/cmTarget.cxx

@@ -64,16 +64,6 @@ const char* cmTarget::GetTargetTypeName(TargetType targetType)
 class cmTargetInternals
 {
 public:
-  cmTargetInternals()
-    : Backtrace()
-    {
-    }
-  cmTargetInternals(cmTargetInternals const&)
-    : Backtrace()
-    {
-    }
-  ~cmTargetInternals();
-
   // The backtrace when the target was created.
   cmListFileBacktrace Backtrace;
 
@@ -94,11 +84,6 @@ public:
   std::vector<cmListFileBacktrace> LinkImplementationPropertyBacktraces;
 };
 
-//----------------------------------------------------------------------------
-cmTargetInternals::~cmTargetInternals()
-{
-}
-
 //----------------------------------------------------------------------------
 cmTarget::cmTarget()
 {