bug 9505 refactor loops in invoices.pl

- Remove an unnecessary loop where output just
  recreated input.
- Remove unnecessary temp variables that obscure code purpose.
- Call the variable containing invoices, invoices
  rather than anonymous and ambiguous results
  reflect namechange in template.
- Lists are passed to template as array refs;
  declare them as scalars as that is how we use them.
- No need to introduce the whole namespace of some C4
  modules for 1 routine.

Test plan:

Note that this patch should not change any visible behavior.

[1] Open the invoice search page.
[2] Verify that the list of suppliers in the drop-down
    on the search form is complete.
[3] Verify that the list of libraries in the drop-down
    on the saerch form is complete.
[4] Perform a search.  Verify that the list of invoices
    is correct.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>