Remove limitation on cross lib dependencies (#106099)

Libs were meant to be a way to break up code from server without
creating full fledged modules. They still exist on the system classpath,
but we did not want to introduce a spaghetti of jars depending on each
other. The check that ensures libs don't depend on each other was added
before Elasticsearch was modularized. Since it now runs modular, the
cross module dependencies are easy to visualize with module-info, and
the module system protects us from circular deps. Additionally, the
number of exceptions to the no-cross-lib-deps rule has grown
considerably.

Given all of the above, the check on cross lib dependencies no longer
provides much benefit, and is more of a hinderance. This commit removes
the check.

libs/build.gradle

@@ -12,35 +12,4 @@ configure(subprojects - project('elasticsearch-log4j')) {
    * tools.
    */
   apply plugin: 'elasticsearch.build'
-
-  /*
-   * Subprojects may depend on the "core" lib but may not depend on any
-   * other libs. This keeps our dependencies simpler.
-   * With the exception that specialised plugin apis can depend on "core" plugin api project
-   */
-  project.afterEvaluate {
-    configurations.all { Configuration conf ->
-      dependencies.matching { it instanceof ProjectDependency }.all { ProjectDependency dep ->
-        Project depProject = dep.dependencyProject
-        if (depProject != null
-          && false == isPluginApi(project, depProject)
-          && false == depProject.path.equals(':libs:elasticsearch-x-content')
-          && false == depProject.path.equals(':libs:elasticsearch-core')
-          && false == depProject.path.equals(':libs:elasticsearch-plugin-api')
-          && false == depProject.path.equals(':libs:elasticsearch-logging')
-          && false == depProject.path.equals(':libs:elasticsearch-native')
-          && depProject.path.startsWith(':libs')
-          && depProject.name.startsWith('elasticsearch-')) {
-          throw new InvalidUserDataException("projects in :libs "
-            + "may not depend on other projects libs except "
-            + ":libs:elasticsearch-core but "
-            + "${project.path} depends on ${depProject.path}")
-        }
-      }
-    }
-  }
-}
-
-boolean isPluginApi(Project project, Project depProject) {
-  return project.path.matches(".*elasticsearch-plugin-.*api")
 }