Remove pointless extension headers

So, I must have been asleep, because I *just* noticed you guys, following polygone's example, created `implement` headers for all the extensions. Those are totally unnecessary, and in fact, some of them (Robert's) had code in them that built but was never run.

I noticed this when I went to fix the problem with paths. Apparently, polygone commented them out sometime around 14BC because the extension system is broken. I created this pull request while fixing the extension system so that you all (A) have a chance to make sure this works before it goes upstream and (B) realize that if something in the extensions framework seems useless, it probably isn't needed.

The only thing this pull request does is delete those headers and the referring lines from the About.ey files. Before I realized Robert's code in two of his `implement.h` files was totally unused, I migrated them to appropriate source files, and so I left them in the commit.

I have tested this request with the default extensions for the ability to compile. Since no code changed, I can't imagine a change in behavior, but feel free to test to your hearts' respective contents.