Remove link to designs.a.o

We now have the designs here, don't point to designs.a.o.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>

content/concepts/_index.md

@@ -14,5 +14,3 @@ These documents cover topics that have been researched but not necessarily imple
 
 Information in these documents may be outdated, but nonetheless provide important context for the decision making process and overall vision of the project.
 
-Many of the concept designs can be found on the [concept design portal](https://designs.apertis.org).
-