Jar plugin loading refactor as generic ServerPlugin

Now that we have a fully pluggable ServerPlugin mechanism
with registered providers, there is no reason why the jar
plugins cannot be loaded using the same interface.

Removing all the custom code managing the jar plugins
and creating a new pre-defined JarPluginProvider.

NOTE: A limitation still exists on init-steps as they
can only be provided by jar plugins. This is due to
the fact that Gerrit init runs without having a fully
configured plugin loader and full set of Guice injectors.

Change-Id: I793570735376be3a40f3204af6e82820191caac6

gerrit-server/src/main/java/com/google/gerrit/server/plugins/PluginModule.java

@@ -31,6 +31,8 @@ public class PluginModule extends LifecycleModule {
     listener().to(PluginLoader.class);
 
     DynamicSet.setOf(binder(), ServerPluginProvider.class);
+    DynamicSet.bind(binder(), ServerPluginProvider.class).to(
+        JarPluginProvider.class);
     bind(UniversalServerPluginProvider.class);
   }
 }