File Discovery: Remove fallback with zen discovery (#26667)

When adding file based discovery, we added a fallback when the discovery
type was set to zen (the default, so everyone got this warning). This
commit removes the fallback for 6.0. Setting file discovery should now
happen explicitly through the hosts_provider setting.

closes #26661

plugins/discovery-file/build.gradle

@@ -52,6 +52,7 @@ setupSeedNodeAndUnicastHostsFile.doLast {
 integTestCluster {
     dependsOn setupSeedNodeAndUnicastHostsFile
     clusterName = 'discovery-file-test-cluster'
+    setting 'discovery.zen.hosts_provider', 'file'
     extraConfigFile 'discovery-file/unicast_hosts.txt', srcUnicastHostsFile
 }
 

plugins/discovery-file/src/main/java/org/elasticsearch/discovery/file/FileBasedDiscoveryPlugin.java

@@ -102,20 +102,4 @@ public Map<String, Supplier<UnicastHostsProvider>> getZenHostsProviders(Transpor
             () -> new FileBasedUnicastHostsProvider(
                     new Environment(settings, configPath), transportService, fileBasedDiscoveryExecutorService));
     }
-
-    @Override
-    public Settings additionalSettings() {
-        // For 5.0, the hosts provider was "zen", but this was before the discovery.zen.hosts_provider
-        // setting existed. This check looks for the legacy zen, and sets the file hosts provider if not set
-        String discoveryType = DiscoveryModule.DISCOVERY_TYPE_SETTING.get(settings);
-        if (discoveryType.equals("zen")) {
-            deprecationLogger.deprecated("Using " + DiscoveryModule.DISCOVERY_TYPE_SETTING.getKey() +
-                " setting to set hosts provider is deprecated. " +
-                "Set \"" + DiscoveryModule.DISCOVERY_HOSTS_PROVIDER_SETTING.getKey() + ": file\" instead");
-            if (DiscoveryModule.DISCOVERY_HOSTS_PROVIDER_SETTING.exists(settings) == false) {
-                return Settings.builder().put(DiscoveryModule.DISCOVERY_HOSTS_PROVIDER_SETTING.getKey(), "file").build();
-            }
-        }
-        return Settings.EMPTY;
-    }
 }