[CSSPGO] Retire FlattenProfileForMatching

- Always use flattened profile to find the profile anchors. Since profile under different contexts may have different inlined callsites, to get more profile anchors, we use a merged profile from all the contexts(the flattened profile) to find callsite anchors.

- Compute the staleness metrics based on the original nested profile, as currently once a callsite is mismatched, all its children profile are dropped.(TODO: in future, we can improve to reuse the children valid profile)

Reviewed By: hoy, wenlei

Differential Revision: https://reviews.llvm.org/D158891