Update the mocked app to cleaner state

[Jeffwan]

Pull Request Description

1. Remove useless files, users should always follow the top level README.md to install the control plan, this is the mocked app folder and people should leverage mocked app from there rather than any other aibrix components. This is good for consistency as well.

2. Change app namespace from aibrix-system to default. The old deployment in aibrix-system hides a lots of edge cases and issues, in real world envs, we won’t deploy data plane workloads in control plane namespace. Change to default to mock the real case.

3. Update README.md structure and make it cleaner to adopt above changes

Related Issues

Resolves: #365

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