Upgrade Swift to 5.0.1, remove unused Docker targets

buildkite/docker/Dockerfile

diff --git a/buildkite/docker/Dockerfile b/buildkite/docker/Dockerfile
index 080e6dc..37098db 100644
--- a/buildkite/docker/Dockerfile
+++ b/buildkite/docker/Dockerfile
@@ -154,7 +154,7 @@
 ENV SWIFT_HOME "/opt/swift"
 ENV PATH "${PATH}:${SWIFT_HOME}/usr/bin"
 RUN mkdir -p /opt/swift && \
-    curl -L https://swift.org/builds/swift-4.2.1-release/ubuntu1404/swift-4.2.1-RELEASE/swift-4.2.1-RELEASE-ubuntu14.04.tar.gz | \
+    curl -L https://swift.org/builds/swift-5.0.1-release/ubuntu1404/swift-5.0.1-RELEASE/swift-5.0.1-RELEASE-ubuntu14.04.tar.gz | \
     tar xz -C /opt/swift --strip 1
 
 USER bazel
@@ -269,7 +269,7 @@
 ENV SWIFT_HOME "/opt/swift"
 ENV PATH "${PATH}:${SWIFT_HOME}/usr/bin"
 RUN mkdir -p /opt/swift && \
-    curl -L https://swift.org/builds/swift-4.2.1-release/ubuntu1604/swift-4.2.1-RELEASE/swift-4.2.1-RELEASE-ubuntu16.04.tar.gz | \
+    curl -L https://swift.org/builds/swift-5.0.1-release/ubuntu1604/swift-5.0.1-RELEASE/swift-5.0.1-RELEASE-ubuntu16.04.tar.gz | \
     tar xz -C /opt/swift --strip 1
 
 USER bazel
@@ -365,38 +365,13 @@
 ENV SWIFT_HOME "/opt/swift"
 ENV PATH "${PATH}:${SWIFT_HOME}/usr/bin"
 RUN mkdir -p /opt/swift && \
-    curl -L https://swift.org/builds/swift-4.2.1-release/ubuntu1804/swift-4.2.1-RELEASE/swift-4.2.1-RELEASE-ubuntu18.04.tar.gz | \
+    curl -L https://swift.org/builds/swift-5.0.1-release/ubuntu1804/swift-5.0.1-RELEASE/swift-5.0.1-RELEASE-ubuntu18.04.tar.gz | \
     tar xz -C /opt/swift --strip 1
 
 USER bazel
 RUN gcloud auth configure-docker --quiet
 RUN bazel version
 
-FROM ubuntu1804-nojava AS ubuntu1804-java8
-USER root
-RUN apt-get -qqy update && \
-    apt-get -qqy install openjdk-8-jdk && \
-    rm -rf /var/lib/apt/lists/*
-USER bazel
-
-FROM ubuntu1804-nojava AS ubuntu1804-java9
-USER root
-RUN apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv 0x219BD9C9 && \
-    apt-add-repository 'deb http://repos.azulsystems.com/ubuntu stable main' && \
-    apt-get -qqy update && \
-    apt-get -qqy install zulu-9 && \
-    rm -rf /var/lib/apt/lists/*
-USER bazel
-
-FROM ubuntu1804-nojava AS ubuntu1804-java10
-USER root
-RUN apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv 0x219BD9C9 && \
-    apt-add-repository 'deb http://repos.azulsystems.com/ubuntu stable main' && \
-    apt-get -qqy update && \
-    apt-get -qqy install zulu-10 && \
-    rm -rf /var/lib/apt/lists/*
-USER bazel
-
 FROM ubuntu1804-nojava AS ubuntu1804-java11
 USER root
 RUN apt-get -qqy update && \