Remove obsolete code from gateway watchers

[y9v]

This :monitor event that we are adding to the response in every gateway watcher is not used anywhere. We are only looking for :block events in the method callers and ignoring the rest.

What does this PR do?
Removes dead code in gateway watchers for different integrations.

Motivation:
Simplifying AppSec code.

Change log entry
None.

Additional Notes:
Here you can check how the response from Gateway#watch is being used:
https://github.com/DataDog/dd-trace-rb/blob/master/lib/datadog/appsec/contrib/rack/request_middleware.rb#L80-L81
https://github.com/DataDog/dd-trace-rb/blob/master/lib/datadog/appsec/contrib/rack/request_middleware.rb#L111-L112
https://github.com/DataDog/dd-trace-rb/blob/master/lib/datadog/appsec/contrib/rails/patcher.rb#L87-L88
https://github.com/DataDog/dd-trace-rb/blob/master/lib/datadog/appsec/contrib/rack/request_body_middleware.rb#L33-L34
https://github.com/DataDog/dd-trace-rb/blob/master/lib/datadog/appsec/contrib/sinatra/patcher.rb#L70-L71
https://github.com/DataDog/dd-trace-rb/blob/master/lib/datadog/appsec/contrib/sinatra/patcher.rb#L111-L112
https://github.com/DataDog/dd-trace-rb/blob/master/lib/datadog/appsec/contrib/graphql/appsec_trace.rb#L24-L25

How to test the change?
CI should be green, and manual testing in app-generator.

[datadog-datadog-prod-us1]

Datadog Report

Branch report: appsec-remove-obsolete-code-from-gateway-watchers
Commit report: 85f6572
Test service: dd-trace-rb

✅ 0 Failed, 22135 Passed, 1476 Skipped, 5m 22.41s Total Time

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-01-09 16:28:30

Comparing candidate commit 85f6572 in PR branch appsec-remove-obsolete-code-from-gateway-watchers with baseline commit bb43112 in branch master.

Found 0 performance improvements and 2 performance regressions! Performance is the same for 29 metrics, 2 unstable metrics.

scenario:profiler - Allocations ()

• 🟥 throughput [-200528.064op/s; -188296.198op/s] or [-5.996%; -5.630%]

scenario:tracing - Propagation - Trace Context

• 🟥 throughput [-3872.210op/s; -3775.047op/s] or [-10.161%; -9.906%]