Remove this code to make field not required.

class-woocommerce-gateway-purchase-order.php

//remove this dumb shit and make PO field not required
//lol $pooorder

	public function validate_fields () {
		$poorder = $this->get_post( 'po_number_field' );
		if( ! $poorder ) {
			if ( function_exists ( 'wc_add_notice' ) ) {
				// Replace this old ass f'in $woocommerce_add_error() function.
				wc_add_notice ( __ ( 'Please enter your PO Number.', 'woocommerce-gateway-purchase-order' ), 'error' );
			} else {
				WC()->add_error( __( 'Please enter your PO Number.', 'woocommerce-gateway-purchase-order' ) );
			}
			return false;
		} else {
			return true;
		}
	} // End the validation stuff