TT#46791 Use proper variable name in test controller_billing.t

The previous variable name didn't represent the real code context.

The changes were initially part of commit a9399b1efd and
were extracted to separate commit as initial commit had been
reverted in commit 0601b821006f.

Change-Id: Idf06a06b60498ec5df7a9472aa38413997dc23cb

t/selenium/controller_billing.t

@@ -164,10 +164,10 @@ $d->scroll_to_element($elem);
 $row = $d->find_element('//div[contains(@class,"dataTables_wrapper")]//td[contains(text(),"2008-02-28")]/..');
 ok($row);
 $d->move_action(element => $row);
-$edit_link = $d->find_child_element($row, './/a[contains(@class,"btn-secondary")]');
+my $delete_button = $d->find_child_element($row, './/a[contains(@class,"btn-secondary")]');
-ok($edit_link);
+ok($delete_button);
 sleep 2 if ($browsername eq "htmlunit");
-$edit_link->click();
+$delete_button->click();
 $d->find_text("Are you sure?");
 $d->find_element('#dataConfirmOK', 'css')->click();