[clang] Revert "Reland "[Clang] Enable -Wunused-template under -Wall" (#208001)" (PR #218638)

[Mariya Podchishchaeva via cfe-commits]

https://github.com/Fznamznon approved this pull request.

I think most of problems reported in the PR are actually fixed though, however if there are false positives, I agree that it makes sense to revert due to release.

AFAIK CI fails are unrelated

https://github.com/llvm/llvm-project/pull/218638