Revert "Include g++-riscv64-linux-gnu in riscv64 cross image"

This reverts commit 2e17531.

Author: zanieb

cpython-unix/build.cross-riscv64.Dockerfile

@@ -57,7 +57,6 @@ RUN apt-get install \
     gcc-powerpc64le-linux-gnu \
     gcc-riscv64-linux-gnu \
     gcc-s390x-linux-gnu \
-    g++-riscv64-linux-gnu \
     libc6-dev-arm64-cross \
     libc6-dev-armel-cross \
     libc6-dev-armhf-cross \