Memory leak in mjs_delete when detaching a body

### Intro

Hi!

I am a PhD student at Zhejiang University. I use MuJoCo for my research on legged locomotion.

I use massively parallel environments for reinforcement learning with procedural terrain generation. After upgrading MuJoCo from 3.3.3 to 3.3.4 (which version replaces `mjs_detachBody` with `mjs_delete`), I found that a continuous memory leak occurs that had never happened before. I checked the source of MuJoCo and found that in version 3.3.3, `mjs_detachBody` calls `model->Detach(body);`, where `subtree->Release();` is invoked.

https://github.com/google-deepmind/mujoco/blob/f978b4eea368450267dd5e8825347653bec7355e/src/user/user_model.cc#L844C1-L850C1

However, after version 3.3.4, the implementation is moved to `void mjCModel::operator-=(mjsElement* el)`, where the `subtree->Release();` is removed. If I manually add `subtree->Release();`, the memory leak is fixed.

https://github.com/google-deepmind/mujoco/blob/8de86c9944e17efd04a3972a630a79a7da30736e/src/user/user_model.cc#L750C1-L757C6

Do I use the api in the wrong way? Or is this indeed a bug?

### My setup

MuJoCo 3.3.3 - 3.3.6 C version

### What's happening? What did you expect?

Memory leak.

### Steps for reproduction

Simple code for reproduction may be hard to provide.

### Minimal model for reproduction

_No response_

### Code required for reproduction

_No response_

### Confirmations

- [x] I searched the [latest documentation](https://mujoco.readthedocs.io/en/latest/overview.html) thoroughly before posting.
- [x] I searched previous [Issues](https://github.com/google-deepmind/mujoco/issues) and [Discussions](https://github.com/google-deepmind/mujoco/discussions), I am certain this has not been raised before.