Explicitly make the rest of erc-compat.el obsolete

Explicitly make the rest of erc-compat.el obsolete

* lisp/obsolete/erc-compat.el (erc-decode-coding-string)
(erc-encode-coding-string, erc-set-write-file-functions)
(erc-emacs-build-time, erc-replace-match-subexpression-in-string)
(erc-member-if, erc-delete-if, erc-remove-if-not, erc-subseq):
Explicitly declare obsolete.
(erc-define-minor-mode): Make into obsolete function alias for
'define-minor-mode'.
(erc-user-emacs-directory): Make into obsolete variable alias for
'user-emacs-directory'.