stop writing small shares in old V1 share format

[zooko]

When we extended the immutable share format to allow large shares, we made it continue writing out the old "V1" share format when the shares were small enough to fit therein. This was for backward compatibility in case someone migrated a share file to an old pre-v1.3 storage server or downgraded their storage server to pre-v1.3. We no longer need to support those uses cases--nobody is going to do either of those two things ever again. It might simplify life for future implementors if we start writing out only V2 share format from now on.