Remove tox-xenial

- EOL, difficult to maintain test environment

.buildbot/tox-xenial/Dockerfile

@@ -1,50 +0,0 @@
-FROM ubuntu:xenial
-
-RUN apt-get update
-
-# Common apt packages
-RUN apt-get install -yq --no-install-suggests --no-install-recommends \
-    software-properties-common build-essential libcap-dev libffi-dev \
-    libssl-dev python-all-dev python-pip python-setuptools python3-dev
-
-# wget
-RUN apt-get install -yq --no-install-suggests --no-install-recommends \
-    libgmp-dev m4 pkgconf wget
-
-RUN wget "https://ftp.gnu.org/gnu/nettle/nettle-3.9.1.tar.gz" \
-    && tar -zxf nettle-3.9.1.tar.gz
-
-RUN cd nettle-3.9.1 \
-    && ./configure --disable-openssl --enable-shared \
-    && make && make install
-
-RUN wget "https://www.gnupg.org/ftp/gcrypt/gnutls/v3.6/gnutls-3.6.16.tar.xz" \
-    && tar -Jxf gnutls-3.6.16.tar.xz
-
-RUN apt-get remove -yq libgnutls30
-
-RUN cd gnutls-3.6.16 \
-    && ./configure --prefix=/usr --without-p11-kit \
-    --with-included-libtasn1 --with-included-unistring --without-idn \
-    && make && make install
-
-RUN wget "https://ftp.gnu.org/gnu/wget/wget2-2.1.0.tar.gz" \
-    && tar -zxf wget2-2.1.0.tar.gz
-
-RUN apt-get remove -yq wget
-
-RUN cd wget2-2.1.0 \
-    && ./configure --without-libpsl --prefix=/usr \
-    GNUTLS_CFLAGS=-I/usr/include/gnutls/ GNUTLS_LIBS=-L/usr/lib \
-    && make && make install \
-    && mv /usr/bin/wget2 /usr/bin/wget
-
-RUN wget -O /usr/local/bin/buildbot_entrypoint.sh http://git.bitmessage.org/Bitmessage/buildbot-scripts/raw/branch/master/docker/xenial/entrypoint.sh
-
-RUN pip install --upgrade pip==20.0.1
-RUN pip install --upgrade setuptools
-RUN pip install tox
-
-ADD . .
-
-CMD .buildbot/tox-xenial/test.sh

.buildbot/tox-xenial/test.sh

@@ -1 +0,0 @@
-../tox-bionic/test.sh