Remove `fmtlib` explicit dependency. (#9702)
This PR removes the temporary fmtlib dependency introduced in #9650 (see
issue #9653).
As explained [in this comment][1], the dependency was only added to work
around a build issue that should have been fixed upstream in PyTorch.
Before the `fmtlib` version bump to 12 in PyTorch
(pytorch/pytorch#163441), `fmtlib` headers were exported by default.
After the bump, they stopped being exported, which caused the issue.
That behavior has since been fixed in PyTorch (pytorch/pytorch#164139),
so we can safely remove the explicit fmtlib dependency.
[1]:
https://github.com/pytorch/xla/pull/9650#pullrequestreview-3264207974
