Revert "[Misc] Add FA2 support to ViT MHA layer (#12355)"

[WoosukKwon]

This reverts commit f1fc051, which currently breaks our CI.

It uses the flash_attn_func API which was removed by a concurrent PR (#12093). @LucasWilkinson is working on getting back the API. After that, we can merge the PR again.

[github-actions]

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[DarkLight1337]

Should be fixed by #12435