backstage: remove empty index.js files

these were previously required by the plugin resolution logic,
but haven't been since Tool Kit v4, which resolves a `.toolkitrc.yml`.

i thought we removed these ages ago

Author: apaleslimghost

plugins/backend-heroku-app/index.js

@@ -2,7 +2,6 @@
   "name": "@dotcom-tool-kit/backend-heroku-app",
   "version": "5.0.0",
   "description": "",
-  "main": "index.js",
   "keywords": [],
   "author": "FT.com Platforms Team <platforms-team.customer-products@ft.com>",
   "license": "ISC",

plugins/backend-heroku-app/package.json

@@ -2,7 +2,6 @@
   "name": "@dotcom-tool-kit/backend-serverless-app",
   "version": "5.0.0",
   "description": "",
-  "main": "index.js",
   "keywords": [],
   "author": "FT.com Platforms Team <platforms-team.customer-products@ft.com>",
   "license": "ISC",

plugins/backend-serverless-app/index.js

@@ -2,7 +2,6 @@
   "name": "@dotcom-tool-kit/component",
   "version": "6.0.0",
   "description": "",
-  "main": "index.js",
   "keywords": [],
   "author": "FT.com Platforms Team <platforms-team.customer-products@ft.com>",
   "license": "ISC",

plugins/backend-serverless-app/package.json

@@ -2,7 +2,6 @@
   "name": "@dotcom-tool-kit/frontend-app",
   "version": "5.0.0",
   "description": "",
-  "main": "index.js",
   "keywords": [],
   "author": "FT.com Platforms Team <platforms-team.customer-products@ft.com>",
   "license": "ISC",