Revert unrelated function pointer change

Author: lucic71

cpp2rust/converter/converter.cpp

@@ -1669,11 +1669,7 @@ bool Converter::VisitImplicitCastExpr(clang::ImplicitCastExpr *expr) {
     ConvertEqualsNullPtr(sub_expr);
     break;
   case clang::CastKind::CK_NullToPointer:
-    if (type->isFunctionPointerType()) {
-      StrCat("None");
-    } else {
-      StrCat(keyword_default_);
-    }
+    StrCat(keyword_default_);
     computed_expr_type_ = ComputedExprType::FreshPointer;
     break;
   default: