[MNG-5227] The 'optional' flag of a dependency should be manageable.

o Updated to add missing management of the optional flag to the
  ModelBuilder. The Maven resolver has been managing the optional
  flag since years. This just adds the missing parts to the
  ModelBuilder to make optional flag management consistent when
  building the effective model no matter if resolved or if build.
diff --git a/maven-model-builder/src/main/java/org/apache/maven/model/management/DefaultDependencyManagementInjector.java b/maven-model-builder/src/main/java/org/apache/maven/model/management/DefaultDependencyManagementInjector.java
index 499f355..dc33853 100644
--- a/maven-model-builder/src/main/java/org/apache/maven/model/management/DefaultDependencyManagementInjector.java
+++ b/maven-model-builder/src/main/java/org/apache/maven/model/management/DefaultDependencyManagementInjector.java
@@ -89,13 +89,6 @@
         }
 
         @Override
-        protected void mergeDependency_Optional( Dependency target, Dependency source, boolean sourceDominant,
-                                                 Map<Object, Object> context )
-        {
-            // optional flag is not managed
-        }
-
-        @Override
         protected void mergeDependency_Exclusions( Dependency target, Dependency source, boolean sourceDominant,
                                                    Map<Object, Object> context )
         {