Remove `nocompatible` from .vimrc

If Vim detects a user's .vimrc, compatibility mode is automatically
disabled. In that event, the only way Vim will start in compatibility
mode is if it's started with the `-C` option.

Considering that, putting `nocompatible` in a .vimrc seems like a bad
idea. Using the `-C` option represents a deliberate request, and I don't
want to automatically override it.