Screwdriver: "Execution Engines" doc page has a yahoo-internal voice

[drewfish]

http://docs.screwdriver.cd/about/appendix/execution-engines/

This page reads to me as having been written by someone inside of yahoo. It might be nice to rephrase it so that it reads as coming from a someone involved with screwdriver.cd but not necessarily from within yahoo.

[petey]

Also, really need to remove TODOs

[tkyi]

Removed the page from the documentation since it isn't really user-relevant.
It might be nice to add an execution engines page to our /design folder in order to document design decisions in the future.