Remove XEmacs compat code from erc

Remove XEmacs compat code from erc

* lisp/erc/erc-button.el (button, erc-button-keymap)
(erc-button-setup, erc-button-add-button): Remove XEmacs compat code.

* lisp/erc/erc-compat.el (erc-replace-match-subexpression-in-string):

* lisp/erc/erc-log.el (erc-log-file-coding-system):

* lisp/erc/erc-menu.el (menu):

* lisp/erc/erc-stamp.el (erc-timestamp-use-align-to):

* lisp/erc/erc-track.el (erc-modified-channels-object, track)
(erc-track-mouse-face):

* lisp/erc/erc.el (erc-update-undo-list)
(erc-quit-reason-various, erc-part-reason-various, erc-cmd-SV)
(erc-input-message, erc-get-channel-mode-from-keypress)
(erc-update-mode-line-buffer):