rm coreth dependency in avalanchego

[huahuayu]

Hi, since coreth is treated as a plugin, and people can write other vm as claimed so avalanchego shouldn't have coreth dependency in go.mod, it should be totally dynamic, because add plugin doesn't need to modify avalanchego code.

But this brings a problem, how to debug plugins? ava-labs/coreth#192

[StephenButtolph]

Coreth was inlined into avalanchego at v1.7.4, which is why it is seen in the go.mod.