Stop passing netbsd_process_target to local functions

Switch from target->read_memory to netbsd_nat::read_memory and
cleanup the code.

No functional change.

gdbserver/ChangeLog:

        * netbsd-low.cc (get_dynamic, get_r_debug, read_one_ptr)
        (netbsd_qxfer_libraries_svr4): Remove "target" argument and update.
        (netbsd_process_target::qxfer_libraries_svr4): Update.