[css-nav-1] Remove link to old issue tracker

All open issues have been migrated, no need to keep pointing people to
it.

Author: frivoal

css-nav-1/Overview.bs

@@ -7,7 +7,6 @@ Work Status: exploring
 Group: csswg
 Repository: wicg/spatial-navigation
 ED: https://drafts.csswg.org/css-nav-1/
-Issue Tracking: Old issues (prior to the migration from WICG) https://github.com/WICG/spatial-navigation/issues?q=is%3Aopen+is%3Aissue+label%3Atopic%3Aspec
 Editor: Jihye Hong, LG Electronics, jh.hong@lge.com
 Editor: Florian Rivoal, Invited Expert, https://florian.rivoal.net
 Abstract: This specification defines a general model for navigating the focus using the arrow keys,