unix: remove arm arches from base Dockerfile

We're not using them and probably never will. The multiarch
support is Jessie is really limiting.

Author: indygreg

cpython-unix/base.Dockerfile

@@ -28,8 +28,6 @@ RUN for s in debian_jessie debian_jessie-updates debian-security_jessie/updates;
     ) > /etc/apt/apt.conf.d/99cpython-portable
 
 RUN ( echo 'amd64'; \
-      echo 'armel'; \
-      echo 'armhf'; \
       echo 'i386'; \
     ) > /var/lib/dpkg/arch