Why was John 5:4 omitted from the Bible?

John 5:4 is a verse that is missing from most modern Bible translations. It reads:

For an angel went down at a certain season into the pool, and troubled the water: whosoever then first after the troubling of the water stepped in was made whole of whatsoever disease he had.

Dragonflies as Spiritual Symbols: Revealing Their Biblical Meaning
Unveiling the Identity of the Bible's King of the North