gdb: don't call so_list::clear in free_so

gdb: don't call so_list::clear in free_so

I think this `so.clear ()` call is not useful.

 - so_list::clear deletes some things that now get automatically deleted
   when the so_list gets deleted right after in free_so.
 - so_list::clear resets some scalar fields of so_list, which we don't
   really care about since the so_list gets deleted right after.
 - so_list::clear calls target_so_ops::clear_so, of which there is a
   single implementation, svr4_clear_so.  That implementation just
   resets a field in lm_info_svr4, which we don't care about, as it will
   get deleted when the so_list gets deleted right after.

Change-Id: Ie4d72f2a04a4129e55c460bb5c69bc0af0d12b32
Approved-By: Pedro Alves <pedro@palves.net>
Reviewed-By: Reviewed-By: Lancelot Six <lancelot.six@amd.com>