Consider removing `count` from sparse share splitter

[rootulp]

Context

https://github.com/informalsystems/audit-celestia/issues/20#issuecomment-1278899785

Problem

The sparse share splitter struct contains the field count which needs to be updated when modifying shares in the sparse share splitter. https://github.com/celestiaorg/celestia-app/blob/3598fe28772ed1ad55a0bc0b76d222828d35960f/pkg/shares/split_sparse_shares.go#L17

Proposal

The count field may be removed from the sparse share splitter struct and the count can be computed by taking the length of shares in the share splitter.