test_efi_plugin_plain_systemd-boot: don't set console

test_efi_plugin_plain_systemd-boot: don't set console

This effectively disables the boot time serial console
so that kernel and initrd logs are not visible which
makes debugging boot failures hard. The logs are
not too verbose, around 270 lines, and can be reduced
with kernel side settings if needed. For example
"loglevel=4" on kernel command line reduces the
output to only warning messages.

Signed-off-by: Mikko Rapeli <mikko.rapeli@linaro.org>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>