fedora.conf: Remove deprecated option in ostree(_installer) phases
This option is deprecated and not needed anymore:
```
[WARNING ] Variant-type source repository is deprecated and will be ignored during 'OSTree' phase: Everything
[OSTREE_INSTALLER] [WARNING ] Variant-type source repository is deprecated and will be ignored during 'OSTreeInstaller' phase: None
```
See: https://pagure.io/pungi-fedora/pull-request/1227#comment-196106
Signed-off-by: Timothée Ravier <tim@siosm.fr>