vim: don't show cursorline in quickfix windows

I had no idea that the current line was stridently highlighted in
quickfix listings, because my use of `cursorline` was always overriding
it with something much more subtle.

Before:
https://www.evernote.com/shard/s219/sh/b0b486d9-d127-49c6-9bdc-f68eb2a0b6db/d9812fa1ea912a7760a012b7e409a6e1

After:
https://www.evernote.com/shard/s219/sh/32051f70-16e0-4126-97fd-d7e868208e3d/8cbfb44f02fb99c8a81271d1b48e051f

Mind. Blown.