new-machine-cymru: remove unneeded bits

I did not run these commands on chi-node-10, and the login console works
fine via both the iDRAC and the java console.

howto/new-machine-cymru.md

@@ -199,15 +199,6 @@ GRUB_TERMINAL="serial console"
 GRUB_SERIAL_COMMAND="serial --speed=115200 --unit=0 --word=8 --parity=no --stop=1"
 ```
 
-Then enable the serial console login as well, for good measure:
-
-    systemctl enable serial-getty@ttyS0.service
-
-If you want to test to see if it actually works (hint: it typically
-doesn't, so don't bother too much):
-
-    systemctl start serial-getty@ttyS0.service
-
 ### initramfs boot config
 
 TODO: figure out the best way to setup the initramfs. So far we've