consensus: remove prepare command

[sdbondi]

Prepare command is not necessary.

Prepare asserts that all local validators agree on input pledges. LocalPrepare asserts that all validators have seen the prepare.
However, the last assertion is intrinsic to the hotstuff consensus i.e. a 3 chain guarantees that all have seen and agree to the command and its pledges and know that all have seen the required QCs. In this case, LocalPrepare can prepare and pledge and by committing this command, we already have both assertions. This becomes obvious by looking at the code for LocalPrepare, which does very little.

Similar logic applies to AllPrepared/LocalAccept, if we are proceeding to LocalAccept then AllPrepared is implicit