Fix pom for build-tools (#33300)

Looks like `java-gradle-plugin` reconfigures the pom.
Stop using it since we don't publish to Gradle plugin portal.

buildSrc/build.gradle

@@ -24,15 +24,6 @@ plugins {
   id 'groovy'
 }
 
-gradlePlugin {
-  plugins {
-    simplePlugin {
-      id = 'elasticsearch.clusterformation'
-      implementationClass = 'org.elasticsearch.gradle.clusterformation.ClusterformationPlugin'
-    }
-  }
-}
-
 group = 'org.elasticsearch.gradle'
 
 String minimumGradleVersion = file('src/main/resources/minimumGradleVersion').text.trim()

buildSrc/src/main/resources/META-INF/gradle-plugins/elasticsearch.clusterformation.properties

@@ -0,0 +1 @@
+implementation-class=org.elasticsearch.gradle.clusterformation.ClusterformationPlugin