server: Disable com_tasks().

This server subcommand will break once we terminate the scheduler
before calling the command handler. The command was only a debugging
aid anyway, and the output was not very interesting to begin with. In
particular, it did not include the tasks of the afs process, and
always showed the same three tasks (command, vss and status).

We keep the subcommand for backward compatibility, but this commit
changes it to be a no-op. Scheduled for removal in v0.7.0.