Configuration update.

- Remove some unused Vim plugins.

- Remove Syntastic, which is no longer strictly necessary for YCM.

- Remove YCM, which has enough of an install process that's it's best
  just to fetch it anew when installing.

- Remove Go support vimscript, since vim-go is better.

- Remove the C++11 syntax shim, since Vim 7.4 supports C++11 properly.

- Add an i3 config.

- Minor other config file changes.