Fix mixin being added to launch arguments twice

The loom mixin extension adds the mixin to the launch arguments automatically. Adding it in the launch config causes the argument to be added twice, which breaks launch logic. Adding the argument twice causes the first command line argument after the second mixin addition to be parsed incorrectly and therefore ignored.

build.gradle.kts

@@ -28,7 +28,6 @@ loom {
             property("mixin.debug", "true")
             property("asmhelper.verbose", "true")
             arg("--tweakClass", "org.spongepowered.asm.launch.MixinTweaker")
-            arg("--mixin", "mixins.$modid.json")
         }
     }
     forge {