proposal: Remove github engine

[Xuanwo]

Nearly no one uses the GitHub engines. But we need to maintain it everywhere in our code base. Since the FUSE engine is much more mature now, we don't need a toy engine for demonstration.

So I propose to remove the GitHub engine entirely.

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Tracked at #7287

[BohuTANG]

Agree +1

[sundy-li]

Ok, let's avoid adding too many useless but cool features in the main branch.