k/group: do not serialize key iobuf as iobuf

When serialized a key is already an iobuf. Wrapping it with another
serialization layer is incorrect as it introduces additional size field
in the binary format that deserializer doesn't expect.

Signed-off-by: Michal Maslanka <michal@vectorized.io>

src/v/kafka/server/group.cc

@@ -2393,7 +2393,7 @@ void add_offset_tombstone_record(
       .partition = tp.partition,
     };
     auto kv = serializer.to_kv(offset_metadata_kv{.key = std::move(key)});
-    builder.add_raw_kv(reflection::to_iobuf(std::move(kv.key)), std::nullopt);
+    builder.add_raw_kv(std::move(kv.key), std::nullopt);
 }
 
 void add_group_tombstone_record(
@@ -2404,7 +2404,7 @@ void add_group_tombstone_record(
       .group_id = group,
     };
     auto kv = serializer.to_kv(group_metadata_kv{.key = std::move(key)});
-    builder.add_raw_kv(reflection::to_iobuf(std::move(kv.key)), std::nullopt);
+    builder.add_raw_kv(std::move(kv.key), std::nullopt);
 }
 } // namespace