Revert "Fix illegal address error in Cycles Light Tree when no emitter is selected"

This reverts commit 206ab6437b.

Seems that the illegal address error should be covered elsewhere, but it's not directly
clear where. Revert the commit for further investigation.

intern/cycles/kernel/light/tree.h

@@ -596,7 +596,7 @@ ccl_device int light_tree_cluster_select_emitter(KernelGlobals kg,
     has_importance |= ((importance[0] > 0) << i);
   }
 
-  if (total_importance[0] == 0.0f || selected_index == -1) {
+  if (total_importance[0] == 0.0f) {
     return -1;
   }