Reorganize vimrc directory structure

I am now no longer using the ~/.vim_runtime directory favored by the no
longer used amix-vimrc.
Instead all of my extra vim config files (that don't go directly in my main
vimrc) are inside of ~/.vim, including those that came from amix-vimrc.
Also, since my vimrc setup is now sufficiently different that it would be
confusing to include the README from amix-vimrc, I have removed said file.
Future customization will likely see my vimrc setup drift even further away
from amix-vimrc.