Drop client-side feature detection for issue relationships

Let the API return its own "unsupported" error rather than gating the
relationship mutations behind a client-side IssueRelationships check.

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

pkg/cmd/issue/edit/edit_test.go

@@ -1227,22 +1227,6 @@ func Test_editRun(t *testing.T) {
 			},
 			stdout: "https://github.com/OWNER/REPO/issue/123\n",
 		},
-		{
-			name: "relationships unsupported on GHES",
-			input: &EditOptions{
-				Detector:     &fd.DisabledDetectorMock{},
-				IssueNumbers: []int{123},
-				Interactive:  false,
-				AddBlockedBy: []string{"200"},
-				FetchOptions: func(_ *api.Client, _ ghrepo.Interface, _ *prShared.Editable, _ gh.ProjectsV1Support) error {
-					return nil
-				},
-			},
-			httpStubs: func(t *testing.T, reg *httpmock.Registry) {
-				mockIssueGet(t, reg)
-			},
-			wantErr: true,
-		},
 		{
 			name: "batch edit type",
 			input: &EditOptions{