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.

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

javatests/google/registry/ui/js/registrar/security_settings_test.js

@@ -55,7 +55,6 @@ function setUp() {
     isAdmin: true,
     xsrfToken: test.testXsrfToken,
     clientId: test.testClientId,
-    showPaymentLink: false,
     logoFilename: 'logo.png',
     productName: 'Nomulus',
     integrationEmail: 'integration@example.com',