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: Ib028104778c1d2e85f967856887d75659532e8cc
diff --git a/microdroid/payload/Android.bp b/microdroid/payload/Android.bp
index 8225875..4814a64 100644
--- a/microdroid/payload/Android.bp
+++ b/microdroid/payload/Android.bp
@@ -31,7 +31,6 @@
     protos: ["metadata.proto"],
     source_stem: "microdroid_metadata",
     host_supported: true,
-    use_protobuf3: true,
     apex_available: [
         "com.android.virt",
     ],