gdb/dwarf: remove type_unit_group

gdb/dwarf: remove type_unit_group

The type_unit_group is an indirection between a stmt_list_hash (possible
dwo_unit + line table section offset) and a type_unit_group_unshareable
that provides no real value.  In dwarf2_per_objfile, we maintain a
stmt_list_hash -> type_unit_group mapping, and in dwarf2_per_objfile, we
maintain a type_unit_group_unshareable mapping.  The type_unit_group
type is empty and only exists to have an identity and to be a link
between the two mappings.

This patch changes it so that we have a single stmt_list_hash ->
type_unit_group_unshareable mapping.

Regression tested on Debian 12 amd64 with a bunch of DWARF target
boards.

Change-Id: I9c5778ecb18963f353e9dd058e0f8152f7d8930c
Approved-By: Tom Tromey <tom@tromey.com>