Form is still trying to do validation
$form['#validate'][] = 'client_mod_uc_cart_checkout_validate';


Will only append your function to the list of functions in the validation array.

I'm fairly certain using = array() won't work because that's how you define an array, not how you empty one.

Modified answer from Stack Overflow:


  Use unset, if you need the array later on just instantiate it again.

unset($form['#validate']);
$form['#validate'] = array();