Disable metal shaders that dont cross compile correctly

Gems/Atom/Feature/Common/Assets/Shaders/MorphTargets/MorphTargetCS.shader

@@ -10,6 +10,7 @@
           "type": "Compute"
         }
       ]
-    }
+    },
+    "DisabledRHIBackends": ["metal"]
 
 }

Gems/Atom/Feature/Common/Assets/Shaders/PostProcessing/LuminanceHistogramGenerator.shader

@@ -12,6 +12,7 @@
           "type": "Compute"
         }
       ]
-    }
+    },
+    "DisabledRHIBackends": ["metal"]
 
 }