chore(flask): remove APM logic from Flask WSGI middleware

[emmettbutler]

This pull request moves the APM-specific logic from the Flask WSGI middleware and the Flask *render* wrapper functions to ddtrace.trace_handlers, clarifying the separation of concerns between Contribs (Flask) and Products (APM).

Checklist

• Change(s) are motivated and described in the PR description.
• Testing strategy is described if automated tests are not included in the PR.
• Risk is outlined (performance impact, potential for breakage, maintainability, etc).
• Change is maintainable (easy to change, telemetry, documentation).
• Library release note guidelines are followed. If no release note is required, add label changelog/no-changelog.
• Documentation is included (in-code, generated user docs, public corp docs).
• Backport labels are set (if applicable)

Reviewer Checklist

• Title is accurate.
• No unnecessary changes are introduced.
• Description motivates each change.
• Avoids breaking API changes unless absolutely necessary.
• Testing strategy adequately addresses listed risk(s).
• Change is maintainable (easy to change, telemetry, documentation).
• Release note makes sense to a user of the library.
• Reviewer has explicitly acknowledged and discussed the performance implications of this PR as reported in the benchmarks PR comment.
• Backport labels are set in a manner that is consistent with the release branch maintenance policy

[majorgreys]

While they were not intended as part of the public interface, the functions removed from the patch module are a breaking change. Can we replace these functions with no-ops and include a deprecation warning?

[emmettbutler]

@majorgreys I changed the diff to avoid removing any public functions.