Remove some unused functions and methods

In the case of CKey's destructor, it seems to have been an oversight in
f4d1fc259 not to delete it. At this point, it results in the move
constructors/assignment operators for CKey being deleted, which may have
a performance impact.

src/key.h

@@ -56,11 +56,6 @@ public:
         keydata.resize(32);
     }
 
-    //! Destructor (again necessary because of memlocking).
-    ~CKey()
-    {
-    }
-
     friend bool operator==(const CKey& a, const CKey& b)
     {
         return a.fCompressed == b.fCompressed &&