[Flang] Remove unused triple variable. NFC (#77275)

I'm not sure why we don't get an unused variable warning, but triple
doesn't
seem to be used after 898db11.

Author: lukel97

flang/lib/Frontend/FrontendActions.cpp

@@ -743,9 +743,6 @@ void CodeGenAction::generateLLVMIR() {
 
   MLIRToLLVMPassPipelineConfig config(level, opts);
 
-  const auto targetOpts = ci.getInvocation().getTargetOpts();
-  const llvm::Triple triple(targetOpts.triple);
-
   if (auto vsr = getVScaleRange(ci)) {
     config.VScaleMin = vsr->first;
     config.VScaleMax = vsr->second;