Remove DISCLAIMER

[wwbmmm]

What problem does this PR solve?

Issue Number: 2078

Problem Summary: Since bRPC has graduated, the DISCLAIMER file can be removed.

What is changed and the side effects?

Changed: Remove DISCLAIMER

Side effects: No

• Performance effects(性能影响): No

• Breaking backward compatibility(向后兼容性): No

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Check List:

• Please make sure your changes are compilable(请确保你的更改可以通过编译).
• When providing us with a new feature, it is best to add related tests(如果你向我们增加一个新的功能, 请添加相关测试).
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