chore: remove go-libp2p-core from go.mod

[marten-seemann]

Having (the deprecated) go-libp2p-core in go.mod causes problems when updating to go-libp2p v0.24.0 and higher (due to incompatible types). To be able to do so in lotus (and other consumers), it can't import any package that references go-libp2p-core.

It was possible to remove the indirect dependency in go.mod without go mod tidy complaining. I had to re-add it as an indirect dependency in ./testplans/graphsync, but we don't care about that package when importing go-graphsync.

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