.shippable.yml: disable powerpc-cross image

Something has happened to the old emdebian setup which means it no
longer builds. Let's disable the shippable builds which are always
failing.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>

.shippable.yml

@@ -23,8 +23,6 @@ env:
       TARGET_LIST=mips-softmmu,mipsel-linux-user
     - IMAGE=debian-mips64el-cross
       TARGET_LIST=mips64el-softmmu,mips64el-linux-user
-    - IMAGE=debian-powerpc-cross
-      TARGET_LIST=ppc-softmmu,ppcemb-softmmu,ppc-linux-user
     - IMAGE=debian-ppc64el-cross
       TARGET_LIST=ppc64-softmmu,ppc64-linux-user,ppc64abi32-linux-user
 build: