Tax total for fees are already flat and calculated

The fee tax was already a float and no longer an array, so flattening it made it calculate wrong. The fee tax totals never got added up to the tax total. This commit fixes that.

Potentially solves #4620 (cc @niklashogefjord)

Small plugin to replicate the issue:

```php
<?php

/*
Plugin Name: WC Fees API test 
*/

add_action( 'woocommerce_before_calculate_totals', 'wc_fees_api_calculate_totals', 10, 1 );

function wc_fees_api_calculate_totals( $totals ) {
	global $woocommerce;
	$woocommerce->cart->add_fee('test fee', 25, true );
}
```