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: I1bfbca5d8a30e4e10f03e4e045d6c6ceddb30c87

rust/protobuf_test.go

@@ -28,7 +28,6 @@ func TestRustProtobuf3(t *testing.T) {
 			protos: ["buf.proto", "proto.proto"],
 			crate_name: "rust_proto",
 			source_stem: "buf",
-            use_protobuf3: true,
 			shared_libs: ["libfoo_shared"],
 			static_libs: ["libfoo_static"],
 		}
@@ -77,7 +76,6 @@ func TestRustProtobufInclude(t *testing.T) {
 			protos: ["proto.proto"],
 			crate_name: "rust_proto",
 			source_stem: "proto",
-			use_protobuf3: true,
 			rustlibs: ["librust_exported_proto", "libfoo"],
 		}
 		rust_protobuf {
@@ -85,7 +83,6 @@ func TestRustProtobufInclude(t *testing.T) {
 			protos: ["proto.proto"],
 			crate_name: "rust_exported_proto",
 			source_stem: "exported_proto",
-			use_protobuf3: true,
 			exported_include_dirs: ["proto"]
 		}
 		rust_library {