remove index.js of plugins

packages/plugin-attachment-refs/index.js

@@ -1 +0,0 @@
-module.exports = require('./src/meta');

packages/plugin-lsx/src/index.js

@@ -1 +0,0 @@
-module.exports = require('./meta');

packages/plugin-pukiwiki-like-linker/src/index.js

@@ -1 +0,0 @@
-module.exports = require('./meta');