build(gradle): remove redundant sourceSets block in protobuf-3.0

Plugin 0.10.0 automatically registers generated proto sources into the
corresponding source sets. The manual srcDir addition was required for
plugin 0.8.18 but is now redundant — keeping it on top of the plugin's
own wiring actively breaks source set registration (review comment on #11310).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: bric3

dd-java-agent/instrumentation/protobuf-3.0/build.gradle

@@ -32,12 +32,4 @@ protobuf {
 dependencies {
   compileOnly group: 'com.google.protobuf', name: 'protobuf-java', version: '3.0.0'
   testImplementation group: 'com.google.protobuf', name: 'protobuf-java', version: '3.0.0'
-}
-
-sourceSets {
-  test {
-    java {
-      srcDir layout.buildDirectory.dir("generated/sources/proto/test/java")
-    }
-  }
-}
+}