Bug 18300: [QA Follow-up] Fix return value inconsistency

Bug 18300: [QA Follow-up] Fix return value inconsistency

As noted on bug report 17669, the return values of delete (both singular
and plural), delete_missing and delete_temporary should be consistent.

Removed the if-construction around full_path. We do not need it; in the
very exceptional case that full_path would be empty, we should delete
the record since the file is missing.

Adjusted POD and unit tests accordingly.

Test plan:
Run t/db_dependent/Upload.t

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>