docker: Remove Dockerfile for i386 builder

We aren't using it and it's broken with the specified upstream image, so
remove it outright.

contrib/Dockerfile.builder.i386

@@ -1,70 +0,0 @@
-FROM i386/ubuntu:16.04
-MAINTAINER Christian Decker <decker.christian@gmail.com>
-
-ENV DEBIAN_FRONTEND noninteractive
-ENV BITCOIN_VERSION 0.18.1
-
-WORKDIR /build
-
-RUN apt-get -qq update && \
-    apt-get -qq install --no-install-recommends --allow-unauthenticated -yy \
-	autoconf \
-	automake \
-	clang \
-	cppcheck \
-	docbook-xml \
-	shellcheck \
-	eatmydata \
-	software-properties-common \
-	build-essential \
-	autoconf \
-	locales \
-	libtool \
-	libprotobuf-c-dev \
-	libsqlite3-dev \
-	libgmp-dev \
-	gettext \
-	git \
-	python \
-	python3 \
-	valgrind \
-	net-tools \
-	python3-mako \
-	python3-pip \
-	python3-setuptools \
-	python-pkg-resources \
-	shellcheck \
-	libxml2-utils \
-	wget \
-	xsltproc \
-	zlib1g-dev && \
-	rm -rf /var/lib/apt/lists/*
-
-ENV LANGUAGE=en_US.UTF-8
-ENV LANG=en_US.UTF-8
-ENV LC_ALL=en_US.UTF-8
-RUN locale-gen en_US.UTF-8 && dpkg-reconfigure locales
-
-RUN cd /tmp/ && \
-    wget https://bitcoin.org/bin/bitcoin-core-$BITCOIN_VERSION/bitcoin-$BITCOIN_VERSION-i686-pc-linux-gnu.tar.gz -O bitcoin.tar.gz && \
-    tar -xvzf bitcoin.tar.gz && \
-    mv /tmp/bitcoin-$BITCOIN_VERSION/bin/bitcoin* /usr/local/bin/ && \
-    rm -rf bitcoin.tar.gz /tmp/bitcoin-$BITCOIN_VERSION
-
-RUN pip3 install --upgrade pip && \
-    python3 -m pip install \
-	CherryPy==17.3.0 \
-	Flask==1.0.2 \
-	cheroot==8.2.1 \
-	ephemeral-port-reserve==1.1.0 \
-	flaky==3.4.0 \
-	pytest-benchmark==3.1.1 \
-        pytest-forked==0.2 \
-	pytest-timeout==1.3.3 \
-	pytest-xdist==1.22.2 \
-	pytest==3.8.1 \
-	python-bitcoinlib==0.10.2 \
-	tqdm==4.26.0 \
-	pytest-test-groups==1.0.3 \
-	flake8==3.5.0 \
-	pytest-rerunfailures==3.1