Remove unneeded Dockerfile for end-to-end tests.

tests/end-to-end/Dockerfile

@@ -1,17 +0,0 @@
-FROM b3vis/borgmatic
-
-# Starting from an existing borgmatic image, install the local source on top of
-# it so that we can test any local changes.
-RUN adduser -h /home -D app
-COPY test_requirements.txt /app/
-RUN pip3 install --upgrade --requirement /app/test_requirements.txt
-COPY . /app
-RUN rm -fr /app/.tox /app/.git \
-    && pip3 install --upgrade /app \
-    && chown -R app /app
-
-USER app
-WORKDIR /app
-ENV BORG_CACHE_DIR "/app/.cache/borg"
-
-CMD ["py.test"]