Remove unnecessary GitHub API preview accept header

Pull request reviews have been part of the v3 API [for almost a year now][1].
Specifying `application/vnd.github.v3+json` would work just as well, but since
[that][2] is already [the default][3], it's not necessary to specify it at all.

[1]: https://developer.github.com/changes/2017-05-09-end-black-cat-preview/
[2]: https://github.com/eclipse/egit-github/blob/v4.11.0.201803080745-r/org.eclipse.egit.github.core/src/org/eclipse/egit/github/core/service/GitHubService.java#L52
[3]: https://github.com/eclipse/egit-github/blob/v4.11.0.201803080745-r/org.eclipse.egit.github.core/src/org/eclipse/egit/github/core/client/GitHubClient.java#L249

src/main/java/org/jenkinsci/plugins/pipeline/github/client/ExtendedPullRequestService.java

@@ -19,8 +19,6 @@
  */
 public class ExtendedPullRequestService extends PullRequestService {
 
-    private static final String BLACK_CAT_PREVIEW = "application/vnd.github.black-cat-preview+json";
-
     public ExtendedPullRequestService(final ExtendedGitHubClient client) {
         super(client);
     }
@@ -166,7 +164,6 @@ public PageIterator<User> pageRequestedReviewers(final IRepositoryIdProvider rep
         uri.append("/requested_reviewers");
 
         PagedRequest<User> request = this.createPagedRequest(1, 100);
-        request.setResponseContentType(BLACK_CAT_PREVIEW);
         request.setUri(uri);
         request.setType((new TypeToken<List<User>>(){}).getType());
         return this.createPageIterator(request);
@@ -186,7 +183,7 @@ public void createReviewRequests(final IRepositoryIdProvider repository,
 
         Map<String, Object> params = new HashMap<>();
         params.put("reviewers", reviewers);
-        getClient().post(uri.toString(), params, ExtendedPullRequest.class, BLACK_CAT_PREVIEW);
+        getClient().post(uri.toString(), params, ExtendedPullRequest.class);
     }
 
     public void deleteReviewRequests(final IRepositoryIdProvider repository,
@@ -203,6 +200,6 @@ public void deleteReviewRequests(final IRepositoryIdProvider repository,
 
         Map<String, Object> params = new HashMap<>();
         params.put("reviewers", reviewers);
-        getClient().delete(uri.toString(), params, BLACK_CAT_PREVIEW);
+        getClient().delete(uri.toString(), params);
     }
 }