Remove optimizeForReading overloads from FrozenDictionary/Set

[stephentoub]

We initially added these as a mitigation primarily for construction potentially being super slow. Now that enough optimization has been done to mitigate those performance concerns, we can get rid of the original mitgation and just have the surface area we initially wanted.

Contributes to #77891
Closes #87633

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[ghost]

Tagging subscribers to this area: @dotnet/area-system-collections
See info in area-owners.md if you want to be subscribed.

Issue Details

---

We initially added these as a mitigation primarily for construction potentially being super slow. Now that enough optimization has been done to mitigate those performance concerns, we can get rid of the original mitgation and just have the surface area we initially wanted.

Contributes to #77891
Closes #87633

Author:	stephentoub
Assignees:	-
Labels:	area-System.Collections, new-api-needs-documentation
Milestone:	-

[adamsitnik]

LGTM, !

@stephentoub is there any chance you know the code size difference?

[stephentoub]

is there any chance you know the code size difference?

With improvements since the last time I measured months ago, it's improved a lot. Each <string, TValue> instantiation without all the optimization strategies looks to be adding ~25K. Each <string, TValue> instantiation with optimizeForReading:true looks to be adding ~50K. That's much better than it had been prior to efforts to drive it down, where it was closer to ~100K if I remember correctly.

[ghost]

Added needs-breaking-change-doc-created label because this PR has the breaking-change label.

When you commit this breaking change:

1. Create and link to this PR and the issue a matching issue in the dotnet/docs repo using the breaking change documentation template, then remove this needs-breaking-change-doc-created label.
2. Ask a committer to mail the .NET Breaking Change Notification DL.

Tagging @dotnet/compat for awareness of the breaking change.