fix: remove vestigial dummy mod container from ASM coremod

The coremod's only job is bootstrapping mixins (done in the plugin
constructor); the DummyModContainer "advancedrocketrycore" registered
empty lifecycle handlers and nothing referenced its modid. It only added
a phantom entry to the title-screen "loaded" count without ever becoming
"active", producing the off-by-one reported in dercodeKoenig#71.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: StannisMod

src/main/java/zmaster587/advancedRocketry/asm/AdvancedRocketryPlugin.java

@@ -34,7 +34,7 @@ public String[] getASMTransformerClass() {
 
     @Override
     public String getModContainerClass() {
-        return "zmaster587.advancedRocketry.asm.ModContainer";
+        return null;
     }
 
     @Override