qanda_helper.php: remove doubled size for answer

The doubled size for answer columns in do_array_multiflexi() is wrong. 20% width are missing from the table and if you have the answer table at 100% width the last column will get the additional 20% and it get's to large.
Example:
$answerwidth=20:
$columnswidth=100-($answerwidth*2)=60;
with $numrows = 2;
$cellwidth=$columnswidth/$numrows=60/2 = 30;
=> columns: 20% answer text, 30% row, 30% row
=> 80% total, so the browser adds the missing 20% to the last column
Result: 20% answer text, 30% row, 50% row
I should have created an issue for this...