Remove onbuild images

The Docker project would like us to deprecate those:
https://github.com/docker-library/official-images/issues/2076

5.0.0.100/onbuild/Dockerfile

@@ -1,11 +0,0 @@
-FROM mono:5.0.0.100
-
-MAINTAINER Jo Shields <jo.shields@xamarin.com>
-
-RUN mkdir -p /usr/src/app/source /usr/src/app/build
-WORKDIR /usr/src/app/source
-
-ONBUILD COPY . /usr/src/app/source
-ONBUILD RUN nuget restore -NonInteractive
-ONBUILD RUN xbuild /property:Configuration=Release /property:OutDir=/usr/src/app/build/
-ONBUILD WORKDIR /usr/src/app/build