[BE] Remove not existing mnist mirror

[atalman]

Looks like original source is empty now:
http://yann.lecun.com/exdb/mnist/

Pytorch hosted mirror exist. Hence leaving it as only option.
https://ossci-datasets.s3.amazonaws.com/mnist/

Fixes these errors in pytorch/ci:

C:\actions-runner\_work\pytorch\pytorch>python tools\download_mnist.py --quiet -d C:\actions-runner\_work\pytorch\pytorch\test\cpp\api\mnist 
Downloading http://yann.lecun.com/exdb/mnist/train-images-idx3-ubyte.gz ...
Failed to download (trying next):
HTTP Error 404: Not Found
Downloading https://ossci-datasets.s3.amazonaws.com/mnist/train-images-idx3-ubyte.gz ...
Downloading http://yann.lecun.com/exdb/mnist/train-labels-idx1-ubyte.gz ...
Failed to download (trying next):
HTTP Error 404: Not Found
Downloading https://ossci-datasets.s3.amazonaws.com/mnist/train-labels-idx1-ubyte.gz ...
Downloading http://yann.lecun.com/exdb/mnist/t10k-images-idx3-ubyte.gz ...
Failed to download (trying next):
HTTP Error 404: Not Found
Downloading https://ossci-datasets.s3.amazonaws.com/mnist/t10k-images-idx3-ubyte.gz ...
Downloading http://yann.lecun.com/exdb/mnist/t10k-labels-idx1-ubyte.gz ...
Failed to download (trying next):
HTTP Error 404: Not Found
Downloading https://ossci-datasets.s3.amazonaws.com/mnist/t10k-labels-idx1-ubyte.gz ...

Link to workflow with example:
https://github.com/pytorch/pytorch/actions/runs/18109150240/job/51542177282#step:15:2335

[pytorch-bot]

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[atalman]

@pytorchmergebot merge -f "lint is green"

[pytorchmergebot]

Merge started

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