appdata: Remove mention of Nominatim search terms

As we don't use Nominatim for search anymore,
remove this section from the description.
2 jobs for master in 10 minutes and 19 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #1292296
flatpak
flatpak

00:09:17

 
  Deploy
passed #1292297
nightly

00:01:02