Remove unused variable

This is over-written a couple of lines later in the template so
we don't need it & ergo don't need to assign it either

Note that the paramter is assigned in postProcess for the
receipt so this is just this usage

CRM/Contribute/Form/Contribution.php

@@ -516,11 +516,6 @@ public function buildQuickForm() {
     $this->assign('payNow', FALSE);
     $buildRecurBlock = FALSE;
 
-    // display tax amount on edit contribution page
-    if ($invoicing && $this->_action & CRM_Core_Action::UPDATE && isset($this->_values['tax_amount'])) {
-      $this->assign('totalTaxAmount', $this->_values['tax_amount']);
-    }
-
     if (empty($this->_lineItems) &&
       ($this->_priceSetId || !empty($_POST['price_set_id']))
     ) {

templates/CRM/Contribute/Form/Contribution.tpl

@@ -597,7 +597,6 @@
           // replace all thousandMarker and change the separator to a dot
           totalAmount = totalAmount.replace(thousandMarker,'').replace(separator,'.');
 
-          var totalTaxAmount = '{/literal}{$totalTaxAmount}{literal}';
           var taxAmount = (taxRate/100)*totalAmount;
           taxAmount = isNaN (taxAmount) ? 0:taxAmount;
           var totalTaxAmount = taxAmount + Number(totalAmount);