Remove any hf-transfer related code

[Wauplin]

Now that all repos are xet-enabled and that hf-xet is installed by default, hf-transfer is not used anymore. Let's remove it from codebase and docs in v1.0.

TL;DR:

• remove hf_transfer extra from dependencies
• remove any hf_transfer related code => it'll be impossible to use hf_transfer with huggingface_hub v1.0. This makes the code much cleaner in the upload/download workflows
• hf-xet is the default (was already the case) but can still be disabled with HF_HUB_DISABLE_XET which defaults to python-based file transfer
• if HF_HUB_ENABLE_HF_TRANSFER is set but not HF_XET_HIGH_PERFORMANCE, we warn the user for awareness
• remove xetEnabled logic from repo settings (as per https://github.com/huggingface-internal/moon-landing/pull/15243 -private link-) => all repos are xet-enabled cc @coyotte508
• update docs to remove mentions to hf_transfer (or tell it's deprecated)
• update docs to mention hf-xet is enabled by default (+remove some outdated info)

TODO:

• review docs related to hf-transfer
• review some wording in Xet docs now that Xet is enabled on every repo (e.g. stop mentioning https://huggingface.co/join/xet)
• check with infra team that hf_transfer is not used anymore internally

Note: this is a drawback for platforms for which hf-xet is not available (32 bits, mips, etc) but honestly I think it's more a feature than a bug. We should encourage users in this situation to report to us and we assess what we should do (I don't expect machines on 32bits to have very large bandwidth anyway^^).

[HuggingFaceDocBuilderDev]

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[hanouticelina]

very nice cleaning! thank you