Remove references to the "magic page" and "random page"

[withzombies]

Real software doesn't have a page of random bytes mapped into every process for its entropy. We need to rewrite the challenges that use the "random page" to use a more standard mechanism for randomness.

https://github.com/CyberGrandChallenge/cgc-release-documentation/blob/master/walk-throughs/submitting-a-cb.md#flag-page