[lldb] Remove scoped timer from high firing and fast running ExtractUnitDIENoDwoIfNeeded

Profiles show that `DWARFUnit::ExtractUnitDIENoDwoIfNeeded` is both high firing (tens of thousands of calls) and fast running (15 µs mean).

Timers like this are noise and load for profiling systems, and can be removed.

rdar://100326595

Differential Revision: https://reviews.llvm.org/D134920