s2a: Use new-style syntax for plugins and remove unused deps

There may be more unused deps, but #11527 makes it far too painful for
me to bother to clean it up more.

s2a/build.gradle

@@ -1,22 +1,15 @@
-buildscript {
-  dependencies {
-    classpath 'com.google.gradle:osdetector-gradle-plugin:1.4.0'
-  }
-}
-
 plugins {
     id "java-library"
     id "maven-publish"
 
     id "com.github.johnrengelman.shadow"
+    id "com.google.osdetector"
     id "com.google.protobuf"
     id "ru.vyarus.animalsniffer"
 }
 
 description = "gRPC: S2A"
 
-apply plugin: "com.google.osdetector"
-
 dependencies {
 
     api project(':grpc-api')
@@ -24,7 +17,6 @@ dependencies {
             project(':grpc-protobuf'),
             project(':grpc-core'),
             libraries.protobuf.java,
-            libraries.conscrypt,
             libraries.guava.jre // JRE required by protobuf-java-util from grpclb
     def nettyDependency = implementation project(':grpc-netty')
     compileOnly libraries.javax.annotation
@@ -36,12 +28,7 @@ dependencies {
             project(':grpc-testing'),
             project(':grpc-testing-proto'),
             testFixtures(project(':grpc-core')),
-            libraries.guava,
-            libraries.junit,
-            libraries.mockito.core,
-            libraries.truth,
-            libraries.conscrypt,
-            libraries.netty.transport.epoll
+            libraries.guava
 
     testImplementation 'com.google.truth:truth:1.4.2'
     testImplementation 'com.google.truth.extensions:truth-proto-extension:1.4.2' 
@@ -74,35 +61,10 @@ dependencies {
             classifier = "windows-x86_64"
         }
     }
-    testRuntimeOnly (libraries.netty.transport.epoll) {
-        artifact {
-            classifier = "linux-x86_64"
-        }
-    }
 
     signature libraries.signature.java
 }
 
-tasks.named("compileJava") {
-    dependsOn(tasks.named("generateProto"))
-    //dependsOn(tasks.named("syncGeneratedSourcesmain"))
-}
-
-
-tasks.named("sourcesJar") {
-    dependsOn(tasks.named("generateProto"))
-    //dependsOn(tasks.named("syncGeneratedSourcesmain"))
-}
-
-sourceSets {
-  main {
-    //java.srcDirs += "src/generated/main/java"
-    //java.srcDirs += "src/generated/main/grpc"
-  }
-}
-//println sourceSets.main.java.srcDirs
-//println sourceSets.test.resources.srcDirs
-
 configureProtoCompilation()
 
 tasks.named("javadoc").configure {