Remove mock backend saga

[RichDom2185]

Let's just expect all developers who want to work on non-playground only frontend, to also have a local backend set up.

Benefits:

• Less things to maintain
• Less things to code for when adding new features
• Current mock saga does not fully work anyway (still requires some backend items)

Drawbacks:

• To test (and/or develop for) Source Academy @ X, now one needs to have a backend setup

[RichDom2185]

Can I get your thoughts on this? @sayomaki @martin-henz @chownces @JoelChanZhiYang

[chownces]

I think both are fine. But currently, the mocks are not actively maintained... so...