Drop broken test asserting invalid configs error on nightly

The current behavior, with the default-false workaround in place,
is not able to identify extra members of the `unstable` table, so
it can't error on unexpected members.

I'll add this test back in with a Deserializer refactor to clean up
the extra logic added to CliUnstable.

Author: bearcage

tests/testsuite/config.rs

@@ -1140,26 +1140,6 @@ unstable.print-im-a-teapot = true
     assert_eq!(config.cli_unstable().print_im_a_teapot, false);
 }
 
-#[cargo_test]
-/// Assert that atempting to set an unstable flag that doesn't exist via config
-/// errors out the same as it would on the command line (nightly only)
-fn unstable_invalid_flag_errors_on_nightly() {
-    cargo::core::enable_nightly_features();
-    write_config(
-        "\
-unstable.an-invalid-flag = 'yes'
-",
-    );
-    assert_error(
-        ConfigBuilder::new().build_err().unwrap_err(),
-        "\
-Invalid [unstable] entry in Cargo config
-
-Caused by:
-  unknown `-Z` flag specified: an-invalid-flag",
-    );
-}
-
 #[cargo_test]
 /// Assert that atempting to set an unstable flag that doesn't exist via config
 /// is ignored on stable