Autogenerate refmap field in mixins json and remove empty server field

Not explicitly specifying the refmap here also removes the log entry in dev environments where Mixin complains about missing refmap.

src/main/resources/carpet.mixins.json

@@ -1,6 +1,5 @@
 {
   "required": true,
-  "refmap": "fabric-carpet-refmap.json",
   "package": "carpet.mixins",
   "compatibilityLevel": "JAVA_16",
   "mixins": [
@@ -205,8 +204,6 @@
     "DebugRenderer_scarpetRenderMixin",
     "LevelRenderer_scarpetRenderMixin"
   ],
-  "server": [
-  ],
   "injectors": {
     "defaultRequire": 1
   }