remove legacy package.json field (#125)

Author: jeetiss

package.json

@@ -3,7 +3,6 @@
   "version": "6.2.3",
   "description": "Monitor if a component is inside the viewport, using IntersectionObserver API",
   "main": "dist/react-intersection-observer.cjs.js",
-  "jsnext:main": "dist/react-intersection-observer.esm.js",
   "module": "dist/react-intersection-observer.esm.js",
   "unpkg": "dist/react-intersection-observer.umd.min.js",
   "author": "Daniel Schmidt",