Clean up slash-log state

[insumity]

Problem

The provider module still stores state under SlashLogBytePrefix.SlashLogBytePrefix is used to construct the SlashLogKey that is used in SetSlashLog and GetSlashLog methods. SetSlashLog is used here while GetSlashLog is only used in an integration test, so the fact that we set something but we have no getters to read it makes this unusable. Additionally, the whole slashing for equivocation evidence chunk where SetSlashLog is called is not really used. Equivocation evidence is to be submitted through MsgSubmitConsumerDoubleVoting messages.

This means that we can remove everything associated with SlashLogBytePrefix.