Delete share related code

[rootulp]

Description

Share related code was copied (and refactored) from celestia-core to celestia-app in celestiaorg/celestia-app#627

We'd like to remove the share related code from celestia-core to avoid maintaining two copies of the same code.

[rootulp]

Notes from attempting to resolve this issue on this branch:

1. celestia-core shouldn't need to ComputeShares itself. This was previously used to generate the DataHash but celestia-core should instead use the DataHash returned from celestia-app in ResponsePrepareProposal.
2. celestia-core performs transaction inclusion logic that seems non-trivial to move to celestia-app. Further investigation is needed here to determine how to keep this functionality after deleting the files in chore: move share encoding logic from core -> app celestia-app#627 (which include types and functions used by TxInclusion).

   celestia-core/pkg/prove/proof.go

   Line 20 in 60930ab

   func TxInclusion(codec rsmt2d.Codec, data types.Data, txIndex uint64) (types.TxProof, error) {

[rootulp]

Given celestia-core and celestia-app need to access the share encoding code, we're considering extracting a separate repo for share related code. The new repo can be depended on by celestia-core and celestia-app.

The current plan is to extract a share repo after non-interactive defaults merges into celestia-app. cc @evan-forbes

[rootulp]

Discussed with @evan-forbes :

• We're going to move TxInclusion code from celestia-core to celestia-app (medium-term) or celestia-node (long-term)
• Since share related code was already moved to celestia-app, it seems safe to delete it from celestia-core