signals: remove forward-declare for signals

This eases the transition to a replacement signals implementation

src/interfaces/handler.h

@@ -5,15 +5,11 @@
 #ifndef BITCOIN_INTERFACES_HANDLER_H
 #define BITCOIN_INTERFACES_HANDLER_H
 
+#include <btcsignals.h>
+
 #include <functional>
 #include <memory>
 
-namespace boost {
-namespace signals2 {
-class connection;
-} // namespace signals2
-} // namespace boost
-
 namespace interfaces {
 
 //! Generic interface for managing an event handler or callback function