Remove unused use_protobuf3 flag

This is a No-op.
Protobuf3 is used regardless of this flag since https://android-review.git.corp.google.com/c/platform/build/soong/+/2817733

Test: m rust

Bug: 308790516
Change-Id: Ib84a01b76a2998249a836fb0278602a5229bd242