doc: remove "Code that grows with grace" video from front page
doc: remove "Code that grows with grace" video from front page

It was deleted from Vimeo. I will attempt to get a new version online.

Fixes issue 7297.

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/61740043

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang