remove index-url for 2.7.0 default pip wheel

[ptrblck]

The current install matrix shows the --index-url for all CUDA versions for our latest PyTorch 2.7.0 release:

However, running pip install torch will install the default config: torch==2.7.0+cu126:

pip install torch
...
Collecting nvidia-cuda-nvrtc-cu12==12.6.77 (from torch)
  Using cached nvidia_cuda_nvrtc_cu12-12.6.77-py3-none-manylinux2014_x86_64.whl.metadata (1.5 kB)
Collecting nvidia-cuda-runtime-cu12==12.6.77 (from torch)
  Using cached nvidia_cuda_runtime_cu12-12.6.77-py3-none-manylinux2014_x86_64.manylinux_2_17_x86_64.whl.metadata (1.5 kB)
Collecting nvidia-cuda-cupti-cu12==12.6.80 (from torch)
  Using cached nvidia_cuda_cupti_cu12-12.6.80-py3-none-manylinux2014_x86_64.manylinux_2_17_x86_64.whl.metadata (1.6 kB)
...
Successfully installed nvidia-cublas-cu12-12.6.4.1 nvidia-cuda-cupti-cu12-12.6.80 nvidia-cuda-nvrtc-cu12-12.6.77 nvidia-cuda-runtime-cu12-12.6.77 nvidia-cudnn-cu12-9.5.1.17 nvidia-cufft-cu12-11.3.0.4 nvidia-cufile-cu12-1.11.1.6 nvidia-curand-cu12-10.3.7.77 nvidia-cusolver-cu12-11.7.1.2 nvidia-cusparse-cu12-12.5.4.2 nvidia-nvjitlink-cu12-12.6.85 nvidia-nvtx-cu12-12.6.77 torch-2.7.0

We should thus remove the unnecessary --index-url for 2.7.0+cu126.

CC @atalman @malfet

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[atalman]

@ptrblck this pr is automated, I believe here is the automated one: #1996

[atalman]

I believe it was caused by this hardcoded value, which was fixed in this PR: pytorch/test-infra#6558

[atalman]

closing this one in favor of automated PR