fix: replace non-functional systemd presets with explicit systemctl enable

Presets only apply during package installation, not when files are
COPY'd into the image. This caused libvirtd, podman.socket,
bootc-fetch-apply-updates.timer, and flatpak-preinstall.service
to remain disabled on first boot.

configs/99-custom.preset

@@ -1,7 +0,0 @@
-enable libvirtd.service
-enable virtlogd.service
-enable podman.socket
-enable bluetooth.service
-enable plasma-setup.service
-enable bootc-fetch-apply-updates.timer
-enable flatpak-preinstall.service