[LegacyPM] Remove ThinLTO bitcode writer legacy pass

Using the legacy PM for the optimization pipeline is deprecated and in
the process of being removed. This is a small step in that direction.

For an example of migrating to the new PM:
https://github.com/llvm/llvm-project/commit/853b57fe80bdb9d6b19697e6f2abc734834e9f6d