Fix crash in tests that use the tree model

 - Since UseAlpha material parameter was removed from lighting

jme3-testdata/src/main/resources/Models/Tree/Leaves.j3m

@@ -4,7 +4,6 @@ Material Leaves : Common/MatDefs/Light/Lighting.j3md {
 
     MaterialParameters {
         DiffuseMap : Models/Tree/Leaves.png
-        UseAlpha : true
         AlphaDiscardThreshold : 0.5
         UseMaterialColors : true
         Ambient : .5 .5 .5 .5