chore(wsgi): remove some appsec code from wsgi contrib

[emmettbutler]

This change adjusts the flask_block callback-setting logic to use the Core API rather than the AppSec-specific set_value call it had used previously.

In the case of request blocking, the separation of concerns ideally breaks down as follows. The AppSec Product code in the ddtrace/appsec directory knows how to make a block/don't block decision based on communication with libddwaf. The Wsgi code in ddtrace/contrib knows how to take that blocking decision into account when processing requests.

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

[pr-commenter]

Benchmarks

Comparing candidate commit 2947218 in PR branch emmett.butler/wsgi-no-appsec with baseline commit a0803f9 in branch 1.x.

Found 1 performance improvements and 0 performance regressions! Performance is the same for 93 cases.

scenario:flasksimple-appsec-get

• 🟩 execution_time [-0.317ms; -0.295ms] or [-5.035%; -4.689%]