flatpak: avoid explicitly setting cflags

The runtimes provide a good set of default flags
at /usr/etc/flatpak-builder/defaults.json
which gets automatically exported
1 job for alatiera/flatpak-things in 6 minutes and 39 seconds
Status Job ID Name Coverage
  Flatpak
passed #839502
flatpak
flatpak:master

00:06:39