Delete: Use hasSuperproject to check

Use hasSuperproject to check if a branch is subscribed by other projects
to avoid recalculation.

Change-Id: I30d7796e48b09b48f2cfafc42a7bca2141db283e

src/main/java/com/googlesource/gerrit/plugins/deleteproject/DeletePreconditions.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/deleteproject/DeletePreconditions.java b/src/main/java/com/googlesource/gerrit/plugins/deleteproject/DeletePreconditions.java
index 417c968..de96ec1 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/deleteproject/DeletePreconditions.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/deleteproject/DeletePreconditions.java
@@ -158,7 +158,7 @@
               .collect(toSet());
       SubmoduleOp sub = subOpFactory.create(branches, mergeOp);
       for (BranchNameKey b : branches) {
-        if (!sub.superProjectSubscriptionsForSubmoduleBranch(b).isEmpty()) {
+        if (sub.hasSuperproject(b)) {
           throw new CannotDeleteProjectException("Project is subscribed by other projects.");
         }
       }