Make babel-runtime use regenerator-runtime from npm. (#3494)

The source repository for regenerator-runtime can now be found at
https://github.com/facebook/regenerator/tree/master/packages/regenerator-runtime

This also removes the babel-regenerator-runtime package, now that the only
difference between it and regenerator-runtime has been resolved:
bc8d2d672c

.gitignore

@@ -13,7 +13,6 @@ dist
 /.package.json
 /packages/babel-runtime/core-js
 /packages/babel-runtime/helpers/*.js
-/packages/babel-runtime/regenerator/*.js
 /packages/*/lib
 _babel.github.io
 /tests/.browser-build.js