[Twisted-Python] removing twistedchecker buildbot for the time being

[Glyph Lefkowitz]

Right now it seems the difference-computation logic on the twistedchecker buildbot has just broken completely.  It's introducing useless noise into the build results because it makes every actually-passing build into a big red 'X' on the pull request status page.  I think I'm going to remove it.  Any objections?

-glyph