Fix to avoid error when publishing

The following error happened.

```
$ vsce publish
Publishing msysyamamoto.vscode-fluentd@0.0.4...
Error: The category 'Languages' is not available in language 'en-us'.
```

Author: msysyamamoto

package.json

@@ -17,9 +17,6 @@
     "engines": {
         "vscode": "^1.14.0"
     },
-    "categories": [
-        "Languages"
-    ],
     "contributes": {
         "languages": [
             {