gdb/dwarf: set m_top_level_die directly in read_cutu_die_from_dwo

read_cutu_die_from_dwo currently returns the dwo's top-level DIE through
a parameter.  Following the previous patch, all code paths end up
setting m_top_level_die.  Simplify this by having read_cutu_die_from_dwo
set m_top_level_die directly.  I think it's easier to understand,
because there's one less indirection to follow.

Change-Id: Ib659f1d2e38501a8fe2b5dd0ca2add3ef55e8d60
Approved-By: Tom Tromey <tom@tromey.com>