systemd: Remove instance name usage in a non-template unit file

console-getty.service is not a template unit file (it doesn't have the
@ specifier), so %I doesn't get properly expanded in it. Thus, getty
startup will fail due to invalid options and no getty prompt is launched
on the console.

Fixes:
No getty prompt on boot

Signed-off-by: Francois Gervais <fgervais@distech-controls.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>