remove dead links for v1606 and v3+ upgrade guides (no content)

upgrade/v2106-Developer-Upgrade-Guide.md

 <!-- --- title: Developer Upgrade Guide (OpenFOAM-v2106) -->
 
 [![home](/icons/home.svg "wiki home")](/home)
+[![user](/icons/user.svg "user upgrade")](/upgrade/v2106-User-Upgrade-Guide)
 [![upgrade](/icons/chevrons-up.svg "upgrade guide")][upgrade-guide]
 [![code](/icons/code.svg "coding patterns")][code-patterns]