Removed invalid link to "infrastructure guide"
Removed invalid link to "infrastructure guide"

Link to "infrastructure guide" gave 404, and I could not find the new link. There are other deliverability guides that talk about infrastructure, but not internal SendGrid infrastructure. This is possibly the new link, but I think it's best to remove the link altogether.

https://sendgrid.com/marketing/guide-2019-deliverability-guide/
Whatthefoxsays/docs,sendgrid/docs,Whatthefoxsays/docs,Whatthefoxsays/docs,sendgrid/docs,sendgrid/docs