[clangd] Remove unused private field 'Opts' in UpdateIndexCallbacks (NFC)

/Users/jiefu/llvm-project/clang-tools-extra/clangd/ClangdServer.cpp:167:32: error: private field 'Opts' is not used [-Werror,-Wunused-private-field]
  const ClangdServer::Options &Opts;
                               ^
1 error generated.