Revert "Install the g++ cross toolchains"

This reverts commit a1828ef.

Author: zanieb

cpython-unix/build.cross.Dockerfile

@@ -59,14 +59,6 @@ RUN apt-get install \
     gcc-mipsel-linux-gnu \
     gcc-powerpc64le-linux-gnu \
     gcc-s390x-linux-gnu \
-    g++-aarch64-linux-gnu \
-    g++-arm-linux-gnueabi \
-    g++-arm-linux-gnueabihf \
-    g++-mips-linux-gnu \
-    g++-mips64el-linux-gnuabi64 \
-    g++-mipsel-linux-gnu \
-    g++-powerpc64le-linux-gnu \
-    g++-s390x-linux-gnu \
     libc6-dev-arm64-cross \
     libc6-dev-armel-cross \
     libc6-dev-armhf-cross \