gdb/dwarf: remove die_reader_specs

die_reader_specs is a relic of some past design, today it only serves as
(useless) a base class for cutu_reader.  Remove it and move all its
fields to cutu_reader.

Change-Id: I5d55018eb8c6e0b828ef5d2f6d09b2047d1a5912
Approved-By: Tom Tromey <tom@tromey.com>