Checking for the 'display-line-numbers symbol property is clumsy and
checking for boundp is pointless now that we have a Emacs 25 polyfill
for display-line-numbers-mode.
This removes the various doom-line-number* variables and replaces it
with the Emacs 26 display-line-numbers API, which I've ported to Emacs
25.x (however, it uses nlinum under the hood, and not all of
display-line-numbers options are supported).
