Remove redundant using namespace Fallback

C++ has ADL to find overloads. using namespace does nothing in this
case.

components/fallback/validate.hpp

@@ -24,8 +24,6 @@ namespace Fallback
     };
 
     // Parses and validates a fallback map from boost program_options.
-    // Note: for boost to pick up the validate function, you need to pull in the namespace e.g.
-    // by using namespace Fallback;
     void validate(boost::any& v, std::vector<std::string> const& tokens, FallbackMap*, int);
 }