buildroot

rsyslog: remove non-existent enable/disable systemd options

The configuration flags `--enable-systemd` and `--disable-systemd` do
not exist in rsyslog (and may have never existed; most likely copy-paste
from other packages which have said options); removing the options.

Signed-off-by: James Knight <james.knight@rockwellcollins.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>