ci: remove centos-steam-8 customer runner

This broke since eef0bae3a7 (migration: Remove block migration) but
even after that was addressed it still fails to complete. As it will
shortly be EOL lets to remove the runner definition and the related
ansible setup bits.

We still have centos9 docker images build and test.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240603175328.3823123-2-alex.bennee@linaro.org>
7 files changed