Remove repeated definition in MAISI (#1786)

Fixes # .

### Description
Remove repeated definition in MAISI

### Checks
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [ ] Avoid including large-size files in the PR.
- [ ] Clean up long text outputs from code cells in the notebook.
- [ ] For security purposes, please check the contents and remove any
sensitive info such as user names and private key.
- [ ] Ensure (1) hyperlinks and markdown anchors are working (2) use
relative paths for tutorial repo files (3) put figure and graphs in the
`./figure` folder
- [ ] Notebook runs automatically `./runner.sh -t <path to .ipynb file>`

---------

Signed-off-by: Can-Zhao <volcanofly@gmail.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: Can-Zhao

generation/maisi/configs/config_maisi_diff_model.json

@@ -1,12 +1,4 @@
 {
-    "noise_scheduler": {
-        "_target_": "generative.networks.schedulers.DDPMScheduler",
-        "num_train_timesteps": 1000,
-        "beta_start": 0.0015,
-        "beta_end": 0.0195,
-        "schedule": "scaled_linear_beta",
-        "clip_sample": false
-    },
     "diffusion_unet_train": {
         "batch_size": 1,
         "cache_rate": 0,