tests/acceptance: drop ARMBIAN_ARTIFACTS_CACHED condition for orangepi-pc, cubieboard...

tests/acceptance: drop ARMBIAN_ARTIFACTS_CACHED condition for orangepi-pc, cubieboard tests

Previously the ARMBIAN_ARTIFACTS_CACHED pre-condition was added to allow running
tests that have already existing armbian.com artifacts stored in the local avocado cache,
but do not have working URLs to download a fresh copy.

At this time of writing the URLs for artifacts on the armbian.com server are updated and working.
Any future broken URLs will result in a skipped acceptance test, for example:

 (1/5) tests/acceptance/boot_linux_console.py:BootLinuxConsole.test_arm_orangepi:
  CANCEL: Missing asset https://apt.armbian.com/pool/main/l/linux-4.20.7-sunxi/linux-image-dev-sunxi_5.75_armhf.deb (0.53 s)

This commits removes the ARMBIAN_ARTIFACTS_CACHED pre-condition such that
the acceptance tests for the orangepi-pc and cubieboard machines can run.

Signed-off-by: Niek Linnenbank <nieklinnenbank@gmail.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Message-id: 20210310195820.21950-6-nieklinnenbank@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>