remove duplicate files

[afeld]

I haven't looked into the code too deeply, but I don't totally understand why both of the following are necessary:

• https://github.com/18F/jekyll-get/blob/master/jekyll_get.rb
• https://github.com/18F/jekyll-get/blob/master/_plugins/jekyll_get.rb

Seems like the two are similar, but not identical.