test: Clean up "supports external snapshot" OS list

The ubuntu-stable image has been 24.04 for some time, so drop the
dynamic special case.

We stopped releasing to Fedora 39, so drop that special case.

test/check-machines-snapshots

@@ -243,12 +243,7 @@ class TestMachinesSnapshots(machineslib.VirtualMachinesCase):
         b.wait_visible("#vm-subVmTest1-add-snapshot-button")
 
         # external memory snapshots introduced in libvirt 9.9.0
-        supports_external = not (
-            m.image.startswith("rhel-8") or
-            m.image in ["debian-stable", "ubuntu-2204", "ubuntu-stable", "fedora-39"])
-        # Transitional code while we move ubuntu-stable from 23.10 mantic to 24.04 noble
-        if m.image == "ubuntu-stable" and m.execute(". /etc/os-release; echo $VERSION_ID").strip() == "24.04":
-            supports_external = True
+        supports_external = not (m.image.startswith("rhel-8") or m.image in ["debian-stable", "ubuntu-2204"])
 
         # HACK: deleting external snapshots for non-running VMs is broken https://bugs.debian.org/bug=1061725
         # Work around that by temporarily disabling libvirtd's AppArmor profile. AppArmor isn't installed by