Remove a deprecated feature of GitLab

Author: harupong

book/04-git-server/sections/gitlab.asc

@@ -128,5 +128,5 @@ Each merge request allows a line-by-line discussion of the proposed change (whic
 Both can be assigned to users, or organized into milestones.
 
 This section has focused mainly on the Git-related parts of GitLab, but it's a fairly mature system, and provides many other features that can help your team work together.
-These include project wikis, discussion ``walls'', and system maintenance tools.
+These include project wikis and system maintenance tools.
 One benefit to GitLab is that, once the server is set up and running, you'll rarely need to tweak a configuration file or access the server via SSH; most administration and general usage can be accomplished through the in-browser interface.