build-system/gnu: Remove redundant 'unpack' phase from 'gnu-dist'.

This has become redundant with the standard 'unpack' phase since
17919a5801.

* guix/build/gnu-dist.scm (copy-source): Remove.
(%dist-phases): Remove 'unpack' phase.

guix/build/gnu-dist.scm

@@ -30,9 +30,6 @@ (define-module (guix build gnu-dist)
 ;;;
 ;;; Code:
 
-(define* (copy-source #:key source #:allow-other-keys)
-  (copy-recursively source "."))
-
 (define* (build #:key build-before-dist? make-flags (dist-target "distcheck")
                 #:allow-other-keys
                 #:rest args)
@@ -57,7 +54,6 @@ (define %dist-phases
   (modify-phases %standard-phases
     (delete 'strip)
     (replace 'install install-dist)
-    (replace 'build build)
-    (replace 'unpack copy-source)))
+    (replace 'build build)))
 
 ;;; gnu-dist.scm ends here