remove some remnants from CLI "experimental" config option

Experimental is always enabled (977d3ae046),
and the `Experimental` field in plugin metadata was deprecated in
977d3ae046 and removed in commit
6a50c4f700.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

e2e/internal/fixtures/fixtures.go

@@ -44,8 +44,7 @@ func SetupConfigWithNotaryURL(t *testing.T, path, notaryURL string) fs.Dir {
 			"%s": {
 				"auth": "ZWlhaXM6cGFzc3dvcmQK"
 			}
-		},
-		"experimental": "enabled"
+		}
 	}
 	`, notaryURL)), fs.WithDir("trust", fs.WithDir("private")))
 	return *dir