Remove unused has_gitlab_shell3? method

lib/tasks/gitlab/check.rake

@@ -589,10 +589,6 @@ namespace :gitlab do
     def gitlab_shell_patch_version
       required_gitlab_shell_version.split(".")[2].to_i
     end
-
-    def has_gitlab_shell3?
-      gitlab_shell_version.try(:start_with?, "v3.")
-    end
   end