Use PR model returned from CancelPendingReview.

Previously we were ignoring the new PR model, and instead mutating the current PR model (wrongly) causing the crash in #1847.

Fixes #1847

Author: grokys

src/GitHub.InlineReviews/Services/PullRequestSession.cs

@@ -230,13 +230,8 @@ public async Task CancelReview()
                 throw new InvalidOperationException("There is no pending review to cancel.");
             }
 
-            await service.CancelPendingReview(LocalRepository, PendingReviewId);
-
-            PullRequest.Reviews = PullRequest.Reviews
-                .Where(x => x.Id != PendingReviewId)
-                .ToList();
-
-            await Update(PullRequest);
+            var pullRequest = await service.CancelPendingReview(LocalRepository, PendingReviewId);
+            await Update(pullRequest);
         }
 
         /// <inheritdoc/>