Remove old presentations from phenopackets.org splash page

[cmungall]

http://phenopackets.org/

It looks like someone recently removed a lot of the old stale text from the page, thanks!

There are still links to very out of date presentations:

Processing phenotype data using Phenopackets-API and PXFTools BOSC 2016 Slides Video Phenopackets: Making phenotype profiles FAIR++ for disease diagnosis and discovery Force2016 Slides

I would remove these. Perhaps have some history section tucked away in the github repo docs.

Also there is a non-link phenopackets_org_email_placeholder