[llvm] [llvm] remove unusable llvm.spec.in (PR #96825)

[Tulio Magno Quites Machado Filho via llvm-commits]

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

The code is broken and fails to generate the SRPM.
Even if generated a correct SRPM, it would still fail because the spec file still uses `llvm/configure`.

This is a clear indication nobody has been using it.

LGTM.

https://github.com/llvm/llvm-project/pull/96825