chore: remove CLA texts (#3577)

CONTRIBUTING.md

@@ -9,7 +9,7 @@ You are welcome to contribute code to the UI5 Web Components in order to fix bug
 
 There are three important things to know:
 
-1. You must be aware of the Apache License (which describes contributions) and **accept the Developer Certificate of Origin**. This is common practice in major Open Source projects. To make this process as simple as possible, we are using *[CLA assistant](https://cla-assistant.io/)* for individual contributions. CLA assistant is an open source tool that integrates with GitHub very well and enables a one-click experience for accepting the CLA. For company contributors, special rules apply. See the respective section below for details.
+1. You must be aware of the Apache License (which describes contributions) and **accept the Developer Certificate of Origin**.
 2. Follow our **[Development Conventions and Guidelines](/docs/Guidelines.md)**.
 3. **Not all proposed contributions can be accepted**. Some features may just fit a third-party add-on better. The code must match the overall direction of the UI5 Web Components and improve it. So there should be some "bang for the byte". For most bug fixes this is a given, but a major feature implementation first needs to be discussed with one of the committers. Possibly, one who touched the related code or module recently. The more effort you invest, the better you should clarify in advance whether the contribution will match the project's direction. The best way would be to just open an enhancement ticket in the issue tracker to discuss the feature you plan to implement (make it clear that you intend to contribute). We will then forward the proposal to the respective code owner. This avoids disappointment.