fix: remove dead 'using namespace boost;' from init.cpp

The cpp20 rebase removed all Boost includes from init.cpp but left
this orphaned using-directive. It compiled on Linux daemon (I2P
embedded transitively provides the boost namespace) but broke every
Qt/Windows/macOS build in CI with: 'boost' is not a namespace-name.

src/init.cpp

@@ -63,7 +63,6 @@ static bool InitWarning(const std::string& str);
 #endif
 
 using namespace std;
-using namespace boost;
 namespace fs = std::filesystem;
 
 namespace {