Demo (backend): no need to remove ffmpeg from conda env anymore

In [backend.Dockerfile](https://github.com/facebookresearch/sam2/blob/main/backend.Dockerfile), `ffmpeg` is being removed from the conda environment, but the current base image (`pytorch/pytorch:2.5.1-cuda12.1-cudnn9-runtime`) does not have `ffmpeg` installed in conda.

https://github.com/facebookresearch/sam2/blob/2b90b9f5ceec907a1c18123530e92e794ad901a4/backend.Dockerfile#L35

Log:
```
$ docker run -it pytorch/pytorch:2.5.1-cuda12.1-cudnn9-runtime bash
root@ae3f3e929651:/workspace# /opt/conda/bin/conda list | grep ffmpeg # <-- does not return anything
```

Previously, `pytorch/pytorch:2.3.1-cuda12.1-cudnn8-runtime` was being used (upgraded to 2.5.1 in #486), which had `ffmpeg` installed:
```
$ docker run -it pytorch/pytorch:2.3.1-cuda12.1-cudnn8-runtime bash
root@5ca29f95d1a2:/workspace# /opt/conda/bin/conda list | grep ffmpeg
ffmpeg 4.3 hf484d3e_0 pytorch
```

Removing `rm /opt/conda/bin/ffmpeg &&` resolves the issue.