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

[kadircet]

Summary

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

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

opt<bool> ShowOrigins{
     desc("Show origins of completion items"),
     init(CodeCompleteOptions().ShowOrigins),
     Hidden,
 };
 
 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),
 };
 
 opt<bool> EnableClangTidy{
     "clang-tidy",
     cat(Features),
     desc("Enable clang-tidy diagnostics"),
     init(true),