Bug 889959 - Don't clear compositor when detaching compositables. r=nical

gfx/layers/composite/CompositableHost.h

@@ -265,7 +265,6 @@ public:
         aLayer == mLayer ||
         aFlags & FORCE_DETACH) {
       SetLayer(nullptr);
-      SetCompositor(nullptr);
       mAttached = false;
       mKeepAttached = false;
       if (mBackendData) {