gdb/dwarf: make read_cutu_die_from_dwo a method of cutu_reader

read_cutu_die_from_dwo is only used as a helper to cutu_reader, so make
it a private method of cutu_reader.

Remove the "result_reader" parameter, because it's always "this".

Change-Id: I7df6162137451c160f0e6bf3539569fcb2421eff
Approved-By: Tom Tromey <tom@tromey.com>