Remove CLA reference from contributing document (#5435)

* Remove CLA reference from contributing document

* Update CONTRIBUTING.md

services/horizon/CONTRIBUTING.md

@@ -1,6 +1,3 @@
 # How to contribute
 
-Please read the [Contribution Guide](https://github.com/stellar/docs/blob/master/CONTRIBUTING.md).
-
-Then please [sign the Contributor License Agreement](https://docs.google.com/forms/d/1g7EF6PERciwn7zfmfke5Sir2n10yddGGSXyZsq98tVY/viewform?usp=send_form).
-
+Please read the [Contribution Guide](https://github.com/stellar/.github/blob/master/CONTRIBUTING.md).