Remove fetch.umd.js library from the package

Problem/Motivation

Per the drupal.org git agreement, only GPL-2+ code is allowed to be added to repositories. The fetch library uses the MIT license, therefore it shouldn't be included in the package.

Proposed resolution

Remove fetch.umd.js.
Add instructions on how to load the library using Composer.

Remaining tasks

Make the changes.

User interface changes

API changes

n/a

Data model changes

n/a