Remove reactive engine from AppSec rack instrumentation

[y9v]

What does this PR do?
It removes Datadog::AppSec::Reactive::Engine.

Motivation:
The problem with reactive engine was that it was not used the way it was intended, and therefore only added unnecessary complexity to our code.

Change log entry
None, this is internal change.

Additional Notes:
None.

How to test the change?
CI and manual testing with app generator.

[datadog-datadog-prod-us1]

Datadog Report

Branch report: appsec-remove-reactive-engine
Commit report: 325ffae
Test service: dd-trace-rb

✅ 0 Failed, 22028 Passed, 1476 Skipped, 5m 39.76s Total Time

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 98.78049% with 1 line in your changes missing coverage. Please review.

Project coverage is 97.72%. Comparing base (e1c3be1) to head (325ffae).

Files with missing lines	Patch %	Lines
lib/datadog/appsec/contrib/rack/gateway/watcher.rb	97.36%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4355      +/-   ##
==========================================
- Coverage   97.72%   97.72%   -0.01%     
==========================================
  Files        1368     1348      -20     
  Lines       83014    82401     -613     
  Branches     4221     4195      -26     
==========================================
- Hits        81124    80523     -601     
+ Misses       1890     1878      -12     

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[Strech]

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[pr-commenter]

Benchmarks

Benchmark execution time: 2025-02-06 17:27:37

Comparing candidate commit 325ffae in PR branch appsec-remove-reactive-engine with baseline commit e1c3be1 in branch master.

Found 1 performance improvements and 0 performance regressions! Performance is the same for 30 metrics, 2 unstable metrics.

scenario:profiler - sample timeline=false

• 🟩 throughput [+0.581op/s; +0.588op/s] or [+9.728%; +9.854%]