flatpak: remove devel prefix

We have a different icon for the development build nowdays
3 jobs for master in 64 minutes and 24 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #841810
flatpak
flatpak

00:16:03

passed #841809
flatpak
tests

00:05:10

 
  Deploy
passed #841811
nightly

00:48:20