[lldb] Remove unused ThreadPlan tracer utilities (NFC)

Delete unused `EnableTracer()` and `SetTracer()` functions on `Thread`. By deleting
these, their `ThreadPlan` counterparts also become unused.

Then, by deleting `ThreadPlanStack::EnableTracer`, `EnableSingleStep` becomes unused.
With no more callers to `EnableSingleStep`, the value `m_single_step` is always true and
can be removed as well.

Differential Revision: https://reviews.llvm.org/D96666

GitOrigin-RevId: 22f0aa0d9ebddb004cbf2d0e7c8c015422351d2b
7 files changed