Remove Duplicate Declaration of pandas in `Dockerfile` (#6959)
### Description
This pull request removes the redundant installation of `pandas` from
the `Dockerfile`.
It was previously declared twice, and this update eliminates the
duplicate entry, improving the clarity and maintainability of the
`Dockerfile`.
https://github.com/microsoft/DeepSpeed/blob/018ece5af2d89a11a4a235f81f94496c78b4f990/docker/Dockerfile#L124
https://github.com/microsoft/DeepSpeed/blob/018ece5af2d89a11a4a235f81f94496c78b4f990/docker/Dockerfile#L135
### Changes
Removed the duplicate pandas installation line from the `RUN pip
install` command.