unneeded bits in vimrc

https://github.com/adarsh/dotfiles/blob/master/vimrc#L142-L145

I don't think you need these if you have an up to date gem as he states that he does this for you.

As an additional bonus.
Your tmux conifg lines look a bit different from the ones listed on the README
https://github.com/christoomey/vim-tmux-navigator
