Inliner refactoring: remove the LR sampling mode

Inliner refactoring: remove the LR sampling mode

Remove code that sampled methods and provided data for the linear
regresssion (LR) size model used by the inliner. We plan on preserving
the current model for legacy behavior and creating new and different
models for future work.

Addreses one part of dotnet/coreclr#3371.

Commit migrated from https://github.com/dotnet/coreclr/commit/563910fef1d865f71cde8244ff7287ffecddb332