import: elpa: Don't hardcode default branch to 'master'.

Otherwise, remotes without a branch named 'master' will cause an error when
importing.

* guix/import/elpa (git-repository->origin): Fallback to HEAD instead of the
  'master' branch.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

guix/import/elpa.scm

@@ -259,7 +259,7 @@ RECIPE."
      ((assoc-ref recipe #:commit)
       => (lambda (commit) (cons 'commit commit)))
      (else
-      '(branch . "master"))))
+      '())))
 
   (let-values (((directory commit) (download-git-repository url ref)))
     `(origin