Remove the restriction that all transactions that were in the prepared state at the initial data timestamp be committed before exiting ‘STARTUP2’. There might be no additional work for this after SERVER-39036, since the way this restriction was enforced was by pinning the stable timestamp behind the oldest prepare timestamp (SERVER-35811) and enforcing that we do not leave 'STARTUP2' until the stable timestamp is greater than the initial data timestamp (SERVER-37973).

We will leave SERVER-37973 in place because it is independently useful, since it ensures that a node will not require a full resync if it crashes or goes into rollback after exiting ‘STARTUP2’.

This work should not be started until testing for initial sync for Prepare Support for Transactions is complete, so that we can check that initial sync tests still pass after this change.