Remove the mockbuild target
    
    The target has always been defined in rpkg and here, originally with
    identical code but recently a change in rpkg had not been made here as
    well, so it was lost to the user.
    
    Also, the target was only registered here, not in rpkg, but this recently
    changed.
    
    All in all, the target should be registered and implemented in rpkg, not
    here.