Remove /slug team reviewer shorthand normalization

Team reviewers must be provided as fully qualified org/teamname.
Remove the /slug shorthand that auto-prefixed the repo owner,
as this format is not supported.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: BagToad

pkg/cmd/pr/shared/params.go

@@ -122,10 +122,6 @@ func AddMetadataToIssueParams(client *api.Client, baseRepo ghrepo.Interface, par
 	var teamReviewers []string
 	for _, r := range tb.Reviewers {
 		if strings.ContainsRune(r, '/') {
-			// Normalize /slug shorthand to org/slug using the repo owner
-			if strings.HasPrefix(r, "/") {
-				r = baseRepo.RepoOwner() + r
-			}
 			teamReviewers = append(teamReviewers, r)
 		} else if r == api.CopilotReviewerLogin {
 			botReviewers = append(botReviewers, r)