Remove sibling_die

Remove sibling_die

The sibling_die helper function does not seem to add much value,
considering that many other fields of die_info are directly accessed.
So, this removes it.

gdb/ChangeLog
2020-03-26  Tom Tromey  <tom@tromey.com>

	* dwarf2/read.c (read_import_statement, read_file_scope)
	(read_type_unit_scope, inherit_abstract_dies, read_func_scope)
	(read_lexical_block_scope, read_call_site_scope)
	(dwarf2_get_subprogram_pc_bounds, get_scope_pc_bounds)
	(handle_struct_member_die, process_structure_scope)
	(update_enumeration_type_from_children)
	(process_enumeration_scope, read_array_type, read_common_block)
	(read_namespace, read_module, read_subroutine_type): Update.
	(sibling_die): Remove.