SubmoduleOp: Make search superproject method private

hasSuperproject() is used to check preconditions during project deletion.
Hence, making superProjectSubscriptionsForSubmoduleBranch() private to
hide details of implementation methods and benefit the split of
SubmoduleOp which will happen later in the change series.

Change-Id: I518a0ab3dcbfbce25554e06afba8e8ffc2feb6f5

java/com/google/gerrit/server/submit/SubmoduleOp.java

@@ -364,8 +364,7 @@ public class SubmoduleOp {
     return ret;
   }
 
-  @UsedAt(UsedAt.Project.PLUGIN_DELETE_PROJECT)
-  public Collection<SubmoduleSubscription> superProjectSubscriptionsForSubmoduleBranch(
+  private Collection<SubmoduleSubscription> superProjectSubscriptionsForSubmoduleBranch(
       BranchNameKey srcBranch) throws IOException {
     logger.atFine().log("Calculating possible superprojects for %s", srcBranch);
     Collection<SubmoduleSubscription> ret = new ArrayList<>();