Bug 15422: Correct calculation of holds ratio report

Bug 15422: Correct calculation of holds ratio report

See comment 1 for a detailed explanation of current calculations and
needed calculations

Also removes an unnecessary variable

To test:
 1 - Place 4 holds on a biblio with one item
 2 - go to /cgi-bin/koha/circ/reserveratios.pl (Circulation->Holds
        ratios)
 3 - Run with 'Hold ratio'=3, it says order 1, ok
 4 - Run with HR=4, it says order 1, wrong
 5 - Run with HR=2, it syas order 2, wrong
 6 - Run with HR=.5, it syas order 4, wrong
 7 - Apply patch
 8 - Run with HR=3, order 1, OK
 9 - Run with HR=4, item does not appear (0 to order), OK
10 - Run with HR=2, order 1, OK
11 - Run with HR=.5, order 7, OK

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>