Remove deprecated nornsctl_ping() function

It has been decided to remove the deprectated nornsctl_ping() function from
libnornsctl.so, since an equivalent functionality is now supported using
the new nornsctl_send_command() function and its NORNSCTL_COMMAND_PING
argument.

Therefore, any such invocation:

    norns_error_t rv = nornsctl_ping();

can now be replicated as:

    norns_error_t rv = nornsctl_send_command(NORNSCTL_COMMAND_PING, NULL);