[Cleanup] Remove TARGET_JVM

This PR removes the unused TARGET_JVM and associated files.

**Note:** As this is a huge changeset, I was very careful and checked that the IL code of all generated assemblies is exactly the same as before (except MVID, Timestamp etc.). This ensures that the change is effectively a no-op.

Props to https://github.com/fanf2/unifdef/ for making the ifdef removal a breeze :tada: .