Remove idx package from inspector msggen

Summary:
This was causing a crash in babel:
```
$ babel src --out-dir bin --source-maps
Error: Cannot find module 'babel-plugin-idx' from
'~/fbsource/xplat/js/react-native-github/ReactCommon/hermes/inspector/tools/msggen'
- If you want to resolve "idx", use "module:idx" {
  code: 'MODULE_NOT_FOUND'
}
```
It didn't appear that this module was used, so I deleted it.

Changelog: [Internal]

Reviewed By: neildhar

Differential Revision: D23993272

fbshipit-source-id: dd34f0fc652cb27c87c891ca37d0eba66a19a6cf

ReactCommon/hermes/inspector/tools/msggen/.babelrc

@@ -5,5 +5,4 @@
       }
     }]
   ],
-  "plugins": ["idx"]
 }

ReactCommon/hermes/inspector/tools/msggen/package.json

@@ -33,7 +33,6 @@
   },
   "dependencies": {
     "devtools-protocol": "0.0.730699",
-    "idx": "^2.1.0",
     "yargs": "^14.2.0"
   }
 }

ReactCommon/hermes/inspector/tools/msggen/yarn.lock

@@ -2519,11 +2519,6 @@ iconv-lite@0.4.24:
   dependencies:
     safer-buffer ">= 2.1.2 < 3"
 
-idx@^2.1.0:
-  version "2.5.6"
-  resolved "https://registry.yarnpkg.com/idx/-/idx-2.5.6.tgz#1f824595070100ae9ad585c86db08dc74f83a59d"
-  integrity sha512-WFXLF7JgPytbMgelpRY46nHz5tyDcedJ76pLV+RJWdb8h33bxFq4bdZau38DhNSzk5eVniBf1K3jwfK+Lb5nYA==
-
 ieee754@^1.1.4:
   version "1.1.13"
   resolved "https://registry.yarnpkg.com/ieee754/-/ieee754-1.1.13.tgz#ec168558e95aa181fd87d37f55c32bbcb6708b84"