[clang-tidy][NFC] remove unused field in UnusedUsingDeclsCheck (#123451)

Author: HerrCai0907

clang-tools-extra/clang-tidy/misc/UnusedUsingDeclsCheck.h

@@ -51,7 +51,6 @@ class UnusedUsingDeclsCheck : public ClangTidyCheck {
   std::vector<UsingDeclContext> Contexts;
   llvm::SmallPtrSet<const Decl *, 32> UsingTargetDeclsCache;
 
-  StringRef RawStringHeaderFileExtensions;
   FileExtensionsSet HeaderFileExtensions;
 };