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

upgrade/v1912-User-Upgrade-Guide.md

 <!-- --- title: User Upgrade Guide (OpenFOAM-v1912) -->
 
 [![home](/icons/home.svg "wiki home")](/home)
+[![devel](/icons/user-plus.svg "devel upgrade")](/upgrade/v1912-Developer-Upgrade-Guide)
 [![upgrade](/icons/chevrons-up.svg "upgrade guide")][upgrade-guide]
 [![code](/icons/code.svg "coding patterns")][code-patterns]