refactor: remove unused boost header include in bitcoin-util.cpp

This header was included since the introduction of bitcoin-util in
commit 13762bcc96, but boost was
actually never used (see `git log -S boost ./src/bitcoin-util.cpp`).

src/bitcoin-util.cpp

@@ -22,8 +22,6 @@
 #include <memory>
 #include <thread>
 
-#include <boost/algorithm/string.hpp>
-
 static const int CONTINUE_EXECUTION=-1;
 
 const std::function<std::string(const char*)> G_TRANSLATION_FUN = nullptr;