Remove a method almost duplicated in other method

Change-Id: Idc9bd0cb53696f379fe77b7b30683eb977a597ad

@@ -86,15 +86,6 @@ interface DiscreteResources {
     DiscreteResources add(DiscreteResources other);
 
     /**
-     * Returns a difference set of this instance and the given resources.
-     * Note: This method returns a new instance, not mutate the current intance.
-     *
-     * @param removed resources
-     * @return a new DiscreteResources instance representing a difference set
-     */
-    DiscreteResources remove(Set<DiscreteResource> removed);
-
-    /**
      * Returns all of resources this instance holds.
      *
      * @return all resources

@@ -58,11 +58,6 @@ final class EmptyDiscreteResources implements DiscreteResources {
     }
 
     @Override
-    public DiscreteResources remove(Set<DiscreteResource> removed) {
-        return this;
-    }
-
-    @Override
     public Set<DiscreteResource> values() {
         return ImmutableSet.of();
     }

@@ -108,11 +108,6 @@ final class EncodableDiscreteResources implements DiscreteResources {
     }
 
     @Override
-    public DiscreteResources remove(Set<DiscreteResource> removed) {
-        return of(parent, Sets.difference(values(), removed));
-    }
-
-    @Override
     public Set<DiscreteResource> values() {
         return values.values().stream()
                 .flatMap(x -> x.values(parent.id()).stream())

@@ -79,12 +79,6 @@ final class GenericDiscreteResources implements DiscreteResources {
         return new GenericDiscreteResources(newValues);
     }
 
-    // returns a new instance, not mutate the current instance
-    @Override
-    public DiscreteResources remove(Set<DiscreteResource> removed) {
-        return of(Sets.difference(this.values, removed));
-    }
-
     @Override
     public Set<DiscreteResource> values() {
         // breaks immutability, but intentionally returns the field

@@ -102,7 +102,8 @@ class TransactionalDiscreteResourceSubStore {
             return true;
         }
 
-        DiscreteResources newValues = oldValues.remove(values);
+        DiscreteResources requested = DiscreteResources.of(values);
+        DiscreteResources newValues = oldValues.difference(requested);
         return childMap.replace(key, oldValues, newValues);
     }
 

@@ -88,11 +88,6 @@ final class UnifiedDiscreteResources implements DiscreteResources {
     }
 
     @Override
-    public DiscreteResources remove(Set<DiscreteResource> removed) {
-        return of(Sets.difference(values(), removed));
-    }
-
-    @Override
     public Set<DiscreteResource> values() {
         return Stream.concat(encodables.values().stream(), generics.values().stream())
                 .collect(Collectors.toCollection(LinkedHashSet::new));