image.bbclass: fix systemd_preset_all

image.bbclass: fix systemd_preset_all

Check the existence of systemd before using systemctl to preset units.
This is because even if 'systemd' is in DISTRO_FEATURES, it's possible
that systemd is not even installed. e.g. container-test-image in
meta-selftest layer.

As systemd DEPENDS on systemd-systemctl-native, the existence of systemd
also ensures the existence of systemd-systemctl-native.

This would fix the following test case when using systemd as the init
manager.

  containerimage.ContainerImageTests.test_expected_files

Also remove the IMAGE_EXTRADEPENDS setting, as nothing references this
variable.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>