guix: Strip #:use-module lists.

This was obtained by setting up this environment:

  guix shell -D guix --with-input=guile@3.0.9=guile-next \
    --with-commit=guile-next=e2ed33ef0445c867fe56c247054aa67e834861f2
    -- make -j5

then adding 'unused-module' to (@@ (guix build compiler) %warnings),
building, and checking all the "unused module" warnings and removing
those that were definitely unused.

guix/store/database.scm

@@ -22,7 +22,6 @@
 (define-module (guix store database)
   #:use-module (sqlite3)
   #:use-module (guix config)
-  #:use-module (guix serialization)
   #:use-module (guix store deduplication)
   #:use-module (guix base16)
   #:use-module (guix progress)
@@ -36,7 +35,6 @@
   #:use-module (srfi srfi-26)
   #:use-module (rnrs io ports)
   #:use-module (ice-9 match)
-  #:use-module (system foreign)
   #:export (sql-schema
             %default-database-file
             store-database-file

guix/store/deduplication.scm

@@ -28,9 +28,7 @@
   #:use-module (srfi srfi-11)
   #:use-module (srfi srfi-34)
   #:use-module (srfi srfi-35)
-  #:use-module (rnrs bytevectors)
   #:use-module (rnrs io ports)
-  #:use-module (ice-9 ftw)
   #:use-module (ice-9 match)
   #:use-module (guix serialization)
   #:export (nar-sha256