Remove top bar 'Explain This'

[misaugstad]

Brief description of problem/feature

We have recently added in better training for users on the mission start screens, so it's about time we removed this old training that no one ever clicked on that we haven't updated in years! It will also clean up the interface a bit, possibly making way for new things like showing the speed limit (#3249)! Here's what it looks like right now:

We also have the same text included in the mobile validate page when you click on the info button at the top-right. We don't have comparable training on mobile yet, so we should probably leave it there for now.