gnu: Don't abuse check-btrfs-file-system to scan.

It was never guaranteed to be run for non-root file systems.  It was for
root file systems only due to a bug now fixed.

* gnu/build/file-systems.scm (check-btrfs-file-system): Don't invoke
‘btrfs device scan’ here.
* gnu/system/linux-initrd.scm (raw-initrd): Do so here if any btrfs file
systems are present.

gnu/build/file-systems.scm

@@ -328,8 +328,6 @@ When FORCE? is true, do perform a real check.  This is not recommended!  See
 @uref{https://bugzilla.redhat.com/show_bug.cgi?id=625967#c8}.  If REPAIR is
 false, do not write to DEVICE.  If it's #t, fix any errors found.  Otherwise,
 fix only those considered safe to repair automatically."
-  ;; XXX Why make this conditional on (check? #t) at all?
-  (system* "btrfs" "device" "scan")     ; ignore errors
   (if force?
       (match (status:exit-val
               (apply system* `("btrfs" "check" "--progress"