[clangd][NFC] Update background-index command line description

Summary:
We didn't change this in D64019 just in case we revert it back.
Deleting it now.

Reviewers: hokein, sammccall

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66943

llvm-svn: 370367

clang-tools-extra/clangd/tool/ClangdMain.cpp

@@ -122,8 +122,7 @@ opt<bool> ShowOrigins{
 opt<bool> EnableBackgroundIndex{
     "background-index",
     cat(Features),
-    desc("Index project code in the background and persist index on disk. "
-         "Experimental"),
+    desc("Index project code in the background and persist index on disk."),
     init(true),
 };