Update CONTRIBUTING.md to remove license statatement

[harold-wang]

Update CONTRIBUTING.md to remove license and elastic.co related statement

Contributing to elasticsearch

Elasticsearch is an open source project and we love to receive contributions from our community — you! There are many ways to contribute, from writing tutorials or blog posts, improving the documentation, submitting bug reports and feature requests or writing code which can be incorporated into Elasticsearch itself.

If you want to be rewarded for your contributions, sign up for the Elastic Contributor Program. Each time you
make a valid contribution, you’ll earn points that increase your chances of winning prizes and being recognized as a top contributor.

2. Sign the Contributor License Agreement

   Please make sure you have signed our Contributor License Agreement. We are not asking you to assign copyright to us, but to give us the right to distribute your code without restriction. We ask this of all contributors in order to assure our users of the origin and continuing existence of the code. You only need to sign the CLA once.

[harold-wang]

Done