Revert "Update nfs.md with information on AWS EFS and Burst Credit usage and how to…"

What does this MR do?

Removes info on how to arbitrarily increase EFS volume size (and as a result, IOPS) and info on using FS cache. We've seen way too many issues with EFS and adding these 'workarounds' in many cases won't even make performance acceptable. I think we should remove this as part of making our stance about EFS stronger.

cc/ @stanhu What do you think?