gdb/23712: Remove dw2_add_symbol_to_list

Finally, we can remove dw2_add_symbol_to_list since the wrapper function
originally introduced to catch this multi-language scenario is no longer
needed.  With multi-language dictionaries, we can now support adding
symbols of multiple languages, negating the need for the assertion
entirely.

This patch should now fix gdb/23712 (and symtab/23010).  At least it will
if the NULL buildsym_compunit problem doesn't strike first (see gdb/23773).

gdb/ChangeLog:

	PR gdb/23712
	PR symtab/23010
	* dwarf2read.c (dw2_add_symbol_to_list): Remove.
	(fixup_go_packaging, new_symbol): Use add_symbol_to_list.