Delete unreachable default clause.

The Dart analyzer will soon be changed so that if the `default` clause
of a `switch` statement is determined to be unreachable by the
exhaustiveness checker, a new warning of type
`unreachable_switch_default` will be issued. This parallels the
behavior of the existing `unreachable_switch_case` warning, which is
issued whenever a `case` clause of a `switch` statement is determined
to be unreachable. For details see
dart-lang/sdk#54575.

This PR deletes an unreachable `default` clause from `dart-sass` now,
to avoid a spurious warning when the analyzer change lands.

lib/src/ast/sass/statement/stylesheet.dart

@@ -91,8 +91,6 @@ final class Stylesheet extends ParentStatement<List<Statement>> {
           return Stylesheet.parseScss(contents, url: url, logger: logger);
         case Syntax.css:
           return Stylesheet.parseCss(contents, url: url, logger: logger);
-        default:
-          throw ArgumentError("Unknown syntax $syntax.");
       }
     } on SassException catch (error, stackTrace) {
       var url = error.span.sourceUrl;