Bug 1000170 - Remove reference to nonexistent class. r=nical

gfx/layers/composite/TiledContentHost.h

@@ -47,7 +47,6 @@ class Compositor;
 class ISurfaceAllocator;
 class Layer;
 class ThebesBufferData;
-class TiledThebesLayerComposite;
 struct EffectChain;