Remove @license from all files other than the top-level index.ts

When building using webpack, the default behavior is to retain comments marked with `@license`.

However, since we have this in every single file, this results in the Esri license header being kept in the built output dozens and dozens of times, bloating the build output.

In Hub, we have overridden this behavior for now, but ideally we would simply remove the `@license` directive from all but the top level `index.ts` that exports all the functions. This would result in a build getting the Esri license... but just once for a package.

For reference, `arcgis-rest-js` simply has this in every file

```js
/* Copyright (c) 2018 Environmental Systems Research Institute, Inc.
* Apache-2.0 */
```

That comment is retained in the esm and commonjs build outputs, and the rollup config injects the following:

```js
/* @preserve
* @esri/arcgis-rest-portal - v2.15.0 - Apache-2.0
* Copyright (c) 2017-2020 Esri, Inc.
* Wed Aug 19 2020 11:59:01 GMT-0600 (Mountain Daylight Time)
*/
```