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: Icd38fa3976d93af730c315a112d50591fe9d00c5

fsverity/Android.bp

@@ -44,7 +44,6 @@ rust_protobuf {
     name: "libfsverity_digests_proto_rust",
     crate_name: "fsverity_digests_proto",
     source_stem: "fsverity_digests_proto",
-    use_protobuf3: true,
     protos: [
         "fsverity_digests.proto",
     ],

ondevice-signing/proto/Android.bp

@@ -37,7 +37,6 @@ rust_protobuf {
     crate_name: "odsign_proto",
     protos: ["odsign_info.proto"],
     source_stem: "odsign_proto",
-    use_protobuf3: true,
     host_supported: true,
     apex_available: [
         "com.android.compos",