Allow users to self-request a PR review (#19030)

The review request feature was added in https://github.com/go-gitea/gitea/pull/10756,
where the doer got explicitly excluded from available reviewers. I don't see a
functionality or security related reason to forbid this case.

As shown by GitHubs implementation, it may be useful to self-request a review,
to be reminded oneselves about reviewing, while communicating to team mates that a
review is missing.

Co-authored-by: delvh <dev.lh@web.de>

services/issue/assignee.go

@@ -140,14 +140,6 @@ func IsValidReviewRequest(reviewer, doer *user_model.User, isAdd bool, issue *mo
 			}
 		}
 
-		if doer.ID == reviewer.ID {
-			return models.ErrNotValidReviewRequest{
-				Reason: "doer can't be reviewer",
-				UserID: doer.ID,
-				RepoID: issue.Repo.ID,
-			}
-		}
-
 		if reviewer.ID == issue.PosterID && issue.OriginalAuthorID == 0 {
 			return models.ErrNotValidReviewRequest{
 				Reason: "poster of pr can't be reviewer",