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

tools/aconfig/Android.bp

@@ -44,7 +44,6 @@ rust_protobuf {
     protos: ["protos/aconfig.proto"],
     crate_name: "aconfig_protos",
     source_stem: "aconfig_protos",
-    use_protobuf3: true,
     host_supported: true,
 }