delete target_ops.to_lookup_symbol

delete target_ops.to_lookup_symbol

gdb/ChangeLog:

        * target.h (struct target_ops): Remove to_lookup_symbol field.
        (target_lookup_symbol): Delete macro.
        * target.c (nosymbol, debug_to_lookup_symbol): Delete.
        (update_current_target, setup_target_debug): Remove handling
        of to_lookup_symbol target_ops field.
        * ada-tasks.c (get_known_tasks_addr): Remove use of
        target_lookup_symbol.
        * coffread.c (coff_symtab_read): Likewise.
        * dbxread.c (read_dbx_symtab): Ditto.