[lldb] Removed scoped timer from ReadMemoryFromFileCache

`ReadMemoryFromFileCache` can be called at a high rate, and has fast execution.
Signposts for high rate & brief duration can have a negative impact on tracing;
emitting a high volume signposts can lead to blocking, affecting performance,
and total volume makes human review of the trace harder because of the noise.

Differential Revision: https://reviews.llvm.org/D121226