Delete private="true" from .soy files

private="true" has never done very much unfortunately. There is an alternate
way to mark templates as private by setting 'visibility="private"' which does
get enforced both by the soy compiler and by custom per-language strategies in
the various backends.

Unfortunately, it isn't possible to safely migrate from one to the other since
users may be calling these templates from server side soy which enforces
visibility via a runtime exception.  So they are just being deleted.

LSC: https://docs.google.com/document/d/1aOM_tBmanDQolF8mAuB0y29yB76cmYS5qOVYBAzFzyw/edit#

Tested:
    TAP --sample for global presubmit queue
    []

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=159565458

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

@@ -162,7 +162,7 @@
 
 
 /** Renders an input form row for a specific type of contact. */
-{template .showContact_ private="true"}
+{template .showContact_ visibility="private"}
   {@param contacts: list<map<string, ?>>} /** List of EPP domain:contacts. */
   {@param type: string} /** Type of contact (e.g. admin, tech) */
   {foreach $contact in $contacts}