Bug 25147: AllowSelfCheckReturns is in the wrong system preference section

Bug 25147: AllowSelfCheckReturns is in the wrong system preference section

This patch moves the AllowSelfCheckReturns system preference to the "Self check-in module" section of the Circulation system preferences.

To test:

1) Check that the AllowSelfCheckReturns system preference is currently
in the "Self Checkout" section of the Circulation system preferences.
2) Apply the patch
3) The preference should now be in the "Self check-in module" section of
the Circulation system preferences.

Signed-off-by: Devinim <kohadevinim@devinim.com.tr>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>