[Driver][NFC] Remove unused variable (#2908)

Author: AaronBallman

clang/lib/Driver/Driver.cpp

@@ -716,7 +716,6 @@ void Driver::CreateOffloadingDeviceToolChains(Compilation &C,
     C.addOffloadDeviceToolChain(CudaTC, OFK);
   } else if (IsHIP) {
     const ToolChain *HostTC = C.getSingleOffloadToolChain<Action::OFK_Host>();
-    const llvm::Triple &HostTriple = HostTC->getTriple();
     auto OFK = Action::OFK_HIP;
     llvm::Triple HIPTriple = getHIPOffloadTargetTriple();
     // Use the HIP and host triples as the key into