[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
7 files changed