Delete unused Dockerfile

[intelliot]

High Level Overview of Change

This old Dockerfile is not used anywhere.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactor (non-breaking change that only restructures code)
• Tests (you added tests for code that already exists, or your new feature included in this PR)
• Documentation update
• Chore (no impact to binary, e.g. .gitignore, formatting, dropping support for older tooling)
• Release

API Impact

• Public API: New feature (new methods and/or new fields)
• Public API: Breaking change (in general, breaking changes should only impact the next api_version)
• libxrpl change (any change that may affect libxrpl or dependents of libxrpl)
• Peer protocol change (must be backward compatible or bump the peer protocol version)

[thejohnfreeman]

Adding a late note. I think this was used once to build the Docker image for the doxygen workflow, but this Dockerfile is based on Ubuntu 16.04, while that image is based on Ubuntu 18.04. I think the currently used image was built from an old version of the Dockerfile in my rippled-docker project. The latest versions do not install Doxygen. When the current image stops working for us, it might be best to add another derivative Dockerfile to that project that installs Doxygen. I endorse removing the Dockerfile in this PR as part of the general migration of containers and packaging into a separate project.