src: fix compiler warnings

The warnings in question are:

../src/node.cc:844:13: warning:
unused function 'DebugProcess' [-Wunused-function]
static void DebugProcess(const FunctionCallbackInfo<Value>& args);
            ^
../src/node.cc:845:13: warning:
unused function 'DebugEnd' [-Wunused-function]
static void DebugEnd(const FunctionCallbackInfo<Value>& args);