ci: flatpak: remove meson-args variable

We no longer need to manually keep it in sync with the ci.
2 jobs for master in 36 minutes and 25 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #839516
flatpak
flatpak

00:31:25

 
  Deploy
passed #839517
nightly

00:04:59