[NFC] Remove unused MLIRContext declaration

[bondhugula]

Summary

Remove unused MLIRContext declaration. NFC.

mlir/lib/Support/MlirOptMain.cpp

LogicalResult mlir::MlirOptMain(int argc, char **argv, llvm::StringRef toolName,
   registerPassManagerCLOptions();
   DebugCounter::registerCLOptions();
   PassPipelineCLParser passPipeline("", "Compiler passes to run");
 
   // Build the list of dialects as a header for the --help message.
   std::string helpHeader = (toolName + "\nAvailable Dialects: ").str();
   {
     llvm::raw_string_ostream os(helpHeader);
-    MLIRContext context;
     interleaveComma(registry.getDialectNames(), os,
                     [&](auto name) { os << name; });
   }
   // Parse pass names in main to ensure static initialization completed.
   cl::ParseCommandLineOptions(argc, argv, helpHeader);
 
   if (showDialects) {
     llvm::outs() << "Available Dialects:\n";