tests/docker: remove dead code for linux-user containers

tests/docker: remove dead code for linux-user containers

debian-powerpc-user-cross was the only linux-user powered cross builder
and it was removed in commit 80394ccf21 ("tests/docker: remove
debian-powerpc-user-cross", 2019-09-26). Remove all the infrastructure
around it since it is now unused.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20220401141326.1244422-2-pbonzini@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220419091020.3008144-5-alex.bennee@linaro.org>