Remove unused debug_str_hash_forced from dwarf2out.

    * dwarf2out.c (debug_str_hash_forced): Removed.
    (gen_label_for_indirect_string): Removed.
    (get_debug_string_label): Removed.
    (AT_string_form): Generate label directly.
    (output_indirect_string): Test indirect_string_node for
    DW_FORM_strp instead of checking label and refcount.
    (prune_indirect_string): Removed.
    (prune_unused_types): Don't check debug_str_hash_forced or
    call prune_indirect_string.

From-SVN: r173355