Remove #:use-module (ice-9 custom-ports)

As it's not actually used.

guix-data-service/jobs/load-new-guix-revision.scm

@@ -22,7 +22,6 @@
   #:use-module (srfi srfi-71)
   #:use-module (ice-9 match)
   #:use-module (ice-9 threads)
-  #:use-module (ice-9 custom-ports)
   #:use-module (ice-9 textual-ports)
   #:use-module (ice-9 hash-table)
   #:use-module (rnrs exceptions)