Fix #2557: Remove incorrect assertion from `ReduceNestingTransform`

ILFunctions can appear as statements when the source code has a lambda expression which is immediately discarded.

```
_ = (Action)delegate { ... };
```

ICSharpCode.Decompiler/IL/Transforms/ReduceNestingTransform.cs

@@ -552,8 +552,6 @@ namespace ICSharpCode.Decompiler.IL
 						ComputeStats(caseBlock, ref numStatements, ref maxDepth, currentDepth);
 					break;
 				case ILFunction func:
-					Debug.Assert(!isStatement);
-
 					int bodyStatements = 0;
 					int bodyMaxDepth = maxDepth;
 					ComputeStats(func.Body, ref bodyStatements, ref bodyMaxDepth, currentDepth);