Remove claims of prepare behavior from linearizable reads

As a premise, 5.0 introduced an optimization for sharded transactions to return to the client after the decision was persisted but before the participating shards commit their local storage transactions. That optimization made it incorrect for linearizable reads to ignore prepare conflicts.

I have no evidence we actually change the prepare conflict behavior for linearizable reads, but we do have code that suggests we may have at some point. That code (according to a patch run) can be deleted.