_includes/references.md cleanup

[jnewbery]

The references.md file could be cleaned up in a few ways:

• Remove the deprecated links and move them to the pages that need them (see everything from " onwards in https://raw.githubusercontent.com/bitcoinops/bitcoinops.github.io/798bf8b519ca19af80bce482d767adeda1ce99d0/_includes/references.md)
• Deduplicate links (eg [c-lightning] and [c-lightning repo] are the same link in https://raw.githubusercontent.com/bitcoinops/bitcoinops.github.io/798bf8b519ca19af80bce482d767adeda1ce99d0/_includes/references.md)
• Remove links that are not commonly reused (eg [Bitcoin Core 0.16.2] is only used once)

It should be easy to make these changes and verify that the generated site is not changed.