Delete all Braintree code

We never launched this, don't planning on launching it now anyway, and it's rotted over the past two years anyway.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=202993577

java/google/registry/ui/soy/registrar/Console.soy

@@ -25,7 +25,6 @@
   {@param clientId: string} /** Registrar client identifier. */
   {@param username: string} /** Arbitrary username to display. */
   {@param logoutUrl: string} /** Generated URL for logging out of Google. */
-  {@param showPaymentLink: bool}
   {@param productName: string} /** Name to display for this software product. */
   {@param integrationEmail: string}
   {@param supportEmail: string}
@@ -76,17 +75,12 @@
 
 /** Sidebar nav. Ids on each elt for testing only. */
 {template .navbar_ visibility="private"}
-  {@param showPaymentLink: bool}
   <div id="reg-nav" class="{css('kd-content-sidebar')}">
     <ul id="reg-navlist">
       <li>
         <a href="/registrar#">Home</a>
       <li>
         <a href="/registrar#resources">Resources &amp; billing</a>
-      {if $showPaymentLink}
-        <li>
-          <a href="/registrar#payment">Pay invoice</a>
-      {/if}
       <li>
         <ul>
           <span class="{css('reg-navlist-sub')}">Settings</span>