[nnbd/collection] remove incorrect cast
[nnbd/collection] remove incorrect cast

This turned up when we fixed a strong mode bug in DDC (which used to skip this check):

https://logs.chromium.org/logs/dart/buildbucket/cr-buildbucket.appspot.com/8887565601020813776/+/steps/test_results/0/logs/new_test_failures__logs_/0

The casts look incorrect, so removing.

Change-Id: I6ea45e60a84417fa630bb849020e66bd503b1d00
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/137041
Reviewed-by: Bob Nystrom <afede3a9ebedb865bee78398feff6fbe97068f17@google.com>
Commit-Queue: Vijay Menon <671ecbb18d31a1c038875027dcfe976af959ea9f@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk