[lldb] Minor cleanups to ThreadPlan.h (NFC)

While learning about ThreadPlan, I did a bit of cleanup:

* Remove unused code
* Move functions to protected where applicable
* Remove virtual for functions that are not overridden

Differential Revision: https://reviews.llvm.org/D96277

GitOrigin-RevId: 606c3be85d9252c0b9964bf4c799a947e3d16a8f
2 files changed