Remove unneeded netbsd_add_process()

Currently it does not add any value.

The netbsd_tdesc local variable is no longer needed. Remove it.
The tdesc value is set by the low target now.

gdbserver/ChangeLog:

        * netbsd-low.cc (netbsd_tdesc): Remove.
        (netbsd_add_process): Likewise.
        (netbsd_process_target::create_inferior): Update.