Remove unnecessary configuration of RuboCop

This seemed necessary at some point but removing it doesn't break
RuboCop.
Enable resizing splits with mouse

See:
https://superuser.com/questions/549930/cant-resize-vim-splits-inside-tmux
Remove arrow key mapping

See discussion: https://github.com/thoughtbot/dotfiles/pull/325

The gist of it is that the use of the arrow keys should be discouraged
in general since it requires moving your hands away from the home row.