Remove hand-coded Univalue destructor.
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: da39a3ee5e6b4b0d3255bfef95601890afd80709@martinus

jgarzik/univalue,jgarzik/univalue