Don't be lenient if JarHell is found

Closes #13404

core/src/test/java/org/elasticsearch/bootstrap/BootstrapForTesting.java

@@ -61,13 +61,7 @@ public class BootstrapForTesting {
         try {
             JarHell.checkJarHell();
         } catch (Exception e) {
-            if (Boolean.parseBoolean(System.getProperty("tests.maven"))) {
             throw new RuntimeException("found jar hell in test classpath", e);
-            } else {
-                Loggers.getLogger(BootstrapForTesting.class)
-                    .warn("Your ide or custom test runner has jar hell issues, " +
-                          "you might want to look into that", e);
-            }
         }
 
         // make sure java.io.tmpdir exists always (in case code uses it in a static initializer)