[Parallel] Revert sequential task changes
https://reviews.llvm.org/D148728 introduced `bool Sequential` to unify
`execute` and the old `spawn` without argument. However, sequential
tasks might be executed by any worker thread (non-deterministic),
leading to non-determinism output for ld.lld -z nocombreloc (see
https://reviews.llvm.org/D133003).
In addition, the extra member variables have overhead.
This sequential task has only been used for lld parallel relocation
scanning.
This patch restores the behavior before https://reviews.llvm.org/D148728 .
Fix #105958
Pull Request: https://github.com/llvm/llvm-project/pull/109084
