[FIX] pos_mercury: get rid of debug in template

Been in there since the introduction of the module at 04fb3dfc. Didn't
really affect anything as long as you don't have developer tools open
while you're printing a ticket on the POSBox. This explains why it has
remained unnoticed for this long.

addons/pos_mercury/static/src/xml/pos_mercury.xml

@@ -55,7 +55,6 @@
 
   <t t-extend="XmlReceipt">
     <t t-jquery="t[t-foreach*='paymentlines'][t-as*='line']" t-operation="append">
-      <t t-debug="true"/>
       <t t-if="line.mercury_data">
         <line line-ratio="1">
           <left><pre>  APPROVAL CODE:</pre><t t-esc="line.mercury_auth_code"/></left>