Delete obsolete TestSourcesPlugin

Since we have migrated to Gradle's built-in test fixture support, the
custom TestSourcesPlugin Gradle plugin is now obsolete.

See gh-23550

integration-tests/integration-tests.gradle

@@ -1,7 +1,5 @@
 description = "Spring Integration Tests"
 
-apply plugin: "org.springframework.build.test-sources"
-
 dependencies {
 	testCompile(project(":spring-aop"))
 	testCompile(project(":spring-beans"))