Clean up layout-files

- Change text to symbols
- Remove unused button-definitions

This makes these layout-files easier to understand and edit.

Part-of: <https://gitlab.gnome.org/World/Phosh/squeekboard/-/merge_requests/629>

data/keyboards/de.yaml

@@ -73,7 +73,6 @@ buttons:
         label: "Ää"
     space:
         outline: "spaceline"
-        label: " "
         text: " "
     Return:
         outline: "altline"