Bug 11148: remove two superflous routines from Koha::DateUtils

Bug 11148: remove two superflous routines from Koha::DateUtils

There are 2 useless routines in the Koha::DateUtils
module:output_pref_due and format_sqlduedatetime. We can call
output_pref and format_datetime with dateonly = 0.

format_sqlduedatetime is only used in one place: opac-reserve.pl

Test plan:
1/ Verify on the opac-reserve.pl page that the date is correctly
displayed for for onloan items (you should use the "specific copy"
feature).
2/ Launch prove t/DateUtils.t UT file and verify all UT pass.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Due date on opac-reserve shown correctly. Unit tests pass.
Did a grep on both function names.

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
No references to subs found. Passes koha-qa.pl, t and xt

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>
(cherry picked from commit b9492e73f59439e218e7f657bdfcf67026834311)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>