[clang][examples] Remove unused variable 'key' in LLVMPrintFunctionNames.cpp (NFC)

/llvm-project/clang/examples/LLVMPrintFunctionNames/LLVMPrintFunctionNames.cpp:33:28: error: unused variable 'key' [-Werror,-Wunused-variable]
  static llvm::AnalysisKey key;
                           ^
1 error generated.

Author: DamonFool

clang/examples/LLVMPrintFunctionNames/LLVMPrintFunctionNames.cpp

@@ -29,9 +29,6 @@ namespace {
 class PrintPass final : public llvm::AnalysisInfoMixin<PrintPass> {
   friend struct llvm::AnalysisInfoMixin<PrintPass>;
 
-private:
-  static llvm::AnalysisKey key;
-
 public:
   using Result = llvm::PreservedAnalyses;