remove no longer needed Docker images

src/ci/docker/disabled/dist-armebv7r-none-eabihf/Dockerfile

-FROM ubuntu:16.04
-
-RUN apt-get update && apt-get install -y --no-install-recommends \
-  g++ \
-  make \
-  file \
-  curl \
-  ca-certificates \
-  python2.7 \
-  git \
-  cmake \
-  sudo \
-  xz-utils \
-  bzip2 \
-  libssl-dev \
-  pkg-config
-
-
-COPY scripts/sccache.sh /scripts/
-RUN sh /scripts/sccache.sh
-
-ENV BASE_URL=https://releases.linaro.org/components/toolchain/binaries/latest/armeb-eabi/
-ENV GCC_LINARO=gcc-linaro-7.2.1-2017.11-x86_64_armeb-eabi
-
-RUN curl -sL $BASE_URL/$GCC_LINARO.tar.xz | tar -xJ
-
-ENV PATH=$PATH:/$GCC_LINARO/bin
-
-ENV TARGET=armebv7r-none-eabihf
-
-ENV CC_armebv7r_none_eabihf=armeb-eabi-gcc \
-    CFLAGS_armebv7r_none_eabihf="-march=armv7-r"
-
-ENV RUST_CONFIGURE_ARGS --disable-docs
-
-ENV SCRIPT python2.7 ../x.py dist --target $TARGET

src/ci/docker/disabled/dist-armv7r-none-eabihf/Dockerfile

-FROM ubuntu:16.04
-
-RUN apt-get update && apt-get install -y --no-install-recommends \
-  g++ \
-  make \
-  file \
-  curl \
-  ca-certificates \
-  python2.7 \
-  git \
-  cmake \
-  sudo \
-  xz-utils \
-  bzip2 \
-  libssl-dev \
-  pkg-config
-
-
-COPY scripts/sccache.sh /scripts/
-RUN sh /scripts/sccache.sh
-
-# GNU Arm Embedded Toolchain 7-2018-q2-update (June 27,2018)
-ENV BASE_URL=https://developer.arm.com/-/media/Files/downloads/gnu-rm/
-RUN curl -L $BASE_URL/7-2018q2/gcc-arm-none-eabi-7-2018-q2-update-linux.tar.bz2 | tar -xj
-ENV PATH=$PATH:/gcc-arm-none-eabi-7-2018-q2-update/bin
-
-ENV TARGET=armv7r-none-eabihf
-
-ENV CC_armv7r_none_eabihf=arm-none-eabi-gcc \
-    CFLAGS_armv7r_none_eabihf="-march=armv7-r"
-
-ENV RUST_CONFIGURE_ARGS --disable-docs
-
-ENV SCRIPT python2.7 ../x.py dist --target $TARGET