Remove duplicate ShareProof from app

[rach-id]

Currently, we have 2 definitions of ShareProof, one in core https://github.com/celestiaorg/celestia-core/blob/v0.34.x-celestia/types/share_proof.go and one in app https://github.com/celestiaorg/celestia-app/blob/main/pkg/proof/proof.pb.go. This makes a confusion when using these types downstream and ultimately requires introducing functions to convert one to the other.

Since ShareProof needs to exist in core because it is part of the transaction proof: https://github.com/celestiaorg/celestia-core/blob/dae3ca9d106c9b27dc192202e052ee0b234f645d/rpc/core/types/responses.go#L225-L232, we should remove its definition from app.