[llvm] [memprof] Omit the key length for the call stack table (PR #89510)

[Teresa Johnson via llvm-commits]

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

Is it worth doing the same for the other EmitKeyDataLength implementations in this file (at least under Version >= 2) since they also appear to be uint64_t?

https://github.com/llvm/llvm-project/pull/89510