Remove hand-coded UniValue destructor.

When the hand-written destructor is removed, the compiler will automatically create a proper one, with correct `noexcept`. This allows `std::vector<UniValue>` to be resized without having to copy all elements first, which makes JSON generation of a bitcoin block (as in the benchmark "BlockToJsonVerbose") 25% faster on my machine.

Author: martinus

include/univalue.h

@@ -47,7 +47,6 @@ class UniValue {
         std::string s(val_);
         setStr(s);
     }
-    ~UniValue() {}
 
     void clear();