gnu: guix-data-service: Fix build.

Use the new EMACSLOADPATH, rather than setting GUIX_ENVIRONMENT. This also
allows changing emacs-no-x to emacs-minimal.

* gnu/packages/web.scm (guix-data-service)[arguments]: Remove the
'set-GUIX_ENVIRONMENT phase.
[native-inputs]: Replace emacs-with-modules with emacs-minimal and
emacs-htmlize.