mantle/platform/qemu: drop json formatting from journal output

In my experience the value of this has been little to none. It
mostly just forces us to filter this output through jq -r .MESSAGE

Let's make it just output text like the non-testiso journal.txt files
are today.

mantle/platform/qemu.go

@@ -1645,7 +1645,7 @@ func (builder *QemuBuilder) VirtioJournal(config *conf.Conf, queryArguments stri
 	StandardOutput=file:/dev/virtio-ports/mantlejournal
 	# Wrap in /bin/bash to hack around SELinux
 	# https://bugzilla.redhat.com/show_bug.cgi?id=1942198
-	ExecStart=/usr/bin/bash -c "journalctl -q -b -f -o json --no-tail %s"
+	ExecStart=/usr/bin/bash -c "journalctl -q -b -f --no-tail %s"
 	[Install]
 	RequiredBy=basic.target
 	`, queryArguments)