Bug #6142 - Delete sub CanBookBeReserved and delete function's reference on @EXPORT

Bug #6142 - Delete sub CanBookBeReserved and delete function's reference on @EXPORT

Original-Author:    Alex Arnaud <alex.arnaud@biblibre.com>, Stéphane Delaune <stephane.delaune@biblibre.com>
Original-Signed-off-by: MJ Ray <mjr@phonecoop.coop>
Correct use statement bug found by Ian Walls

Backporting to 3.6.x

Author: MJ Ray <mjr@phonecoop.coop>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>
Signed-off-by: Chris Nighswonger <chris.nighswonger@gmail.com>