Delete two functions obsolete since Emacs 23

Delete two functions obsolete since Emacs 23

These were announced as removed in Emacs 28.1 NEWS, but apparently
never actually deleted: only their obsoletion warning was removed.
So the warnings were reintroduced in Emacs 28.2, and we now actually
delete them here instead.

* src/process.c (Fset_process_filter_multibyte)
(Fprocess_filter_multibyte_p): Delete functions obsolete since
Emacs 23.
(syms_of_process): Delete corresponding defsubrs.
* lisp/subr.el (process-filter-multibyte-p)
(set-process-filter-multibyte): Delete obsoletion.
* lisp/erc/erc-dcc.el (erc-dcc-server): Don't use obsolete functions.