Adapt ProjectBranchesScreen to use REST for listing/deleting branches

This removes the old RPC for deleting branches which is now not used
anymore.

Change-Id: Ib50c61aa7aaff86f5dfe0244b2925d3532018f93
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>

gerrit-gwtui/src/main/java/com/google/gerrit/client/admin/AdminConstants.java

@@ -89,7 +89,6 @@ public interface AdminConstants extends Constants {
   String initialRevision();
   String buttonAddBranch();
   String buttonDeleteBranch();
-  String branchDeletionOpenChanges();
 
   String groupItemHelp();