Remove useless infra assert

Summary: Some product callsite just pass in nil object to this API thus keep causing the assertion. It's a bit too noisy and not much value here since we by default would still return `NSNotFound` here.

Reviewed By: stavash

Differential Revision: D24233480

fbshipit-source-id: 38b2eed3a9286bdd0a7eb0467894a13bbdca007a

Source/IGListKit/Internal/IGListSectionMap.m

@@ -95,7 +95,9 @@
 }
 
 - (NSInteger)sectionForObject:(id)object {
-    IGParameterAssert(object != nil);
+    if (object == nil) {
+        return NSNotFound;
+    }
 
     id sectionController = [self sectionControllerForObject:object];
     if (sectionController == nil) {