Until now, (guix deprecation) had its own warning mechanism, which was
inconsistent (it did not use colors, etc.)
* guix/deprecation.scm (deprecation-warning-port): Remove
(source-properties->location-string): Remove.
(warn-about-deprecation): Use 'warning' instead of 'format'.
(define-deprecated, define-deprecated/alias): Adjust docstring.
* guix/channels.scm (build-from-source): Refer to 'guix-warning-port'
instead of 'deprecation-warning-port'.
69962ab7a8