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

[imprs]

In 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.

sam2/backend.Dockerfile

Line 35 in 2b90b9f

RUN rm /opt/conda/bin/ffmpeg && ln -s /bin/ffmpeg /opt/conda/bin/ffmpeg

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.