Remove invalid head_dim entry

Setting this to 0 is wrong and breaks sane parsing of the config, in particular the conversion script for `llama.cpp`.

config.json

@@ -40,6 +40,5 @@
     "embedding_dropout": 0.1,
     "norm_head": true,
     "norm_softmax": false,
-    "output_dropout": 0.1,
-    "head_dim": 0
+    "output_dropout": 0.1
 }