[Frontend] Remove "SupportedFeatures" section from -emit-supported-features

It was never implemented and we are re-purposing this for compatibility
purposed into `-emit-supported-arguments`

Author: xedin

lib/FrontendTool/FrontendTool.cpp

@@ -1158,10 +1158,6 @@ static bool printSwiftFeature(CompilerInstance &instance) {
 #include "swift/Option/Options.inc"
 #undef OPTION
   out << "    \"LastOption\"\n";
-  out << "  ],\n";
-  out << "  \"SupportedFeatures\": [\n";
-  // Print supported feature names here.
-  out << "    \"LastFeature\"\n";
   out << "  ]\n";
   return false;
 }