refactor: drop unused parent-project registration

Nothing in the build references project(":instrumentation") as a
dependency, and ./gradlew test/build already recurses into all subprojects.
The aggregator hook was redundant.

Author: Copilot

examples/distro/buildSrc/src/main/kotlin/otel.instrumentation-conventions.gradle.kts

@@ -83,13 +83,3 @@ class JavaagentProvider(
 ) : CommandLineArgumentProvider {
   override fun asArguments(): Iterable<String> = listOf("-javaagent:${agentJar.get().absolutePath}")
 }
-
-// Register this instrumentation module with the :instrumentation aggregator project
-// so that ":instrumentation:test" and ":instrumentation:assemble" pull in every
-// instrumentation module without an explicit subprojects {} block.
-project.parent?.let { parent ->
-  parent.tasks.named("test").configure {
-    dependsOn(tasks.named("test"))
-  }
-  parent.dependencies.add("implementation", project)
-}