Despite our attempts in 4d26c7fe to keep this going it still gets in
the way of "make docker-test-build" completing because of course we
can't build a modern QEMU with the image. Let's put the thing out of
its misery and remove it.

People who really care about building on powerpc can still use the
binfmt_misc support to manually build an image (or just run the build
from pre this commit).
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>
Reviewed-by: NRichard Henderson <richard.henderson@linaro.org>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Cc: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>