Remove isVisibleToAll from GroupDescription

The notion of who can view a group is a security detail that
belongs delegated to the group backend and should not be part
of the public GroupDescription.Basic interface.

Drop it and move the code into the internal group backend only.

Change-Id: Ia967e957147860fe98d1a4d74b005d63f48e4343

gerrit-server/src/main/java/com/google/gerrit/server/auth/ldap/LdapGroupBackend.java

@@ -149,11 +149,6 @@ public class LdapGroupBackend implements GroupBackend {
         return name;
       }
 
-      @Override
-      public boolean isVisibleToAll() {
-        return false;
-      }
-
       @Override
       @Nullable
       public String getEmailAddress() {