[clangd] The ClangdServer::EnableHiddenFeatures is not used any more.

Remove it.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@369631 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/clangd/ClangdServer.h b/clangd/ClangdServer.h
index e1620cf..378cea5 100644
--- a/clangd/ClangdServer.h
+++ b/clangd/ClangdServer.h
@@ -324,7 +324,6 @@
   // If this is true, suggest include insertion fixes for diagnostic errors that
   // can be caused by missing includes (e.g. member access in incomplete type).
   bool SuggestMissingIncludes = false;
-  bool EnableHiddenFeatures = false;
 
   std::function<bool(const Tweak &)> TweakFilter;