Partial revert of "Support combine.self='remove'" to fix MNG-7709 (#241)

This reverts commit 89f74c5.

# Conflicts:
#	src/test/java/org/codehaus/plexus/util/xml/Xpp3DomTest.java

Author: gnodet

src/main/java/org/codehaus/plexus/util/xml/Xpp3Dom.java

@@ -445,7 +445,7 @@ private static void mergeIntoXpp3Dom( Xpp3Dom dominant, Xpp3Dom recessive, Boole
             {
                 for ( String attr : recessive.attributes.keySet() )
                 {
-                    if ( isEmpty( dominant.getAttribute( attr ) ) && !SELF_COMBINATION_MODE_ATTRIBUTE.equals( attr ) )
+                    if ( isEmpty( dominant.getAttribute( attr ) ) )
                     {
                         dominant.setAttribute( attr, recessive.getAttribute( attr ) );
                     }