Remove unnecessary registries for 2.7 install instructions

[tengyifei]

It looks like we already got rid of the libtpu-nightly cleanup package in the 2.7 stable branch. Here we remove those -f flags in the README too.

We'll need to make a similar change in the README.md of the master branch at a later time when 2.7 is released.

TESTED:

wget https://storage.googleapis.com/pytorch-xla-releases/wheels/tpuvm/torch_xla-2.7.0rc5-cp311-cp311-linux_x86_64.whl pip install torch_xla-2.7.0rc5-cp311-cp311-linux_x86_64.whl 'torch_xla[tpu]' # No extra registries needed here!

[zpcore]

Thanks for updating the README. Is this because of #9008? Previously we pointed to the wrong libtpu release link.

[tengyifei]

Is this because of #9008? Previously we pointed to the wrong libtpu release link.

Does #9008 fix a particular test/build/workflow? It seems orthogonal to me. Both are useful.

PyTorch/XLA used to depend on libtpu-nightly==0.1.dev20241010+nightly.cleanup, which was used during the migration from the libtpu-nightly package to the libtpu package, and is only installable via custom registries. That dependency is now gone which allows us to remove the -f flags.

[zpcore]

Oh, should we apply the PR to the master branch? I think people will only read the master README.

[tengyifei]

Yes, that's a good idea. 2.7 is not yet published. Probably we could update the README when 2.7 is out so people don't get mislead.