lisp/erc: Use lexical-binding

lisp/erc: Use lexical-binding

Also remove various redundant `:group` arguments.

* lisp/erc/erc-backend.el (define-erc-response-handler): Move `declare`
after the docstring.

* lisp/erc/erc-capab.el: Use lexical-binding.
(erc-capab-identify-activate): Simplify with `member`.

* lisp/erc/erc-dcc.el (erc-dcc): Move before erc-dcc-mode definition,
which refers to it.
(erc-dcc-chat-accept): Remove unused vars `nick` and `buffer`.

* lisp/erc/erc-imenu.el: Use lexical-binding.
(erc-create-imenu-index): Remove unused var `prev-pos`.

* lisp/erc/erc-match.el: Use lexical-binding.
(erc-match-message): Remove unused var `old-pt`.
(erc-match-message): Strength-reduce `eval` to `symbol-value`.

* lisp/erc/erc-page.el: Use lexical-binding.
(erc-page): Move Custom group before `erg-page-mode` which refers to it.

* lisp/erc/erc-replace.el: Use lexical-binding.
(erc-replace-insert): Use `functionp`.

* lisp/erc/erc-status-sidebar.el: Use lexical-binding.
(erc-status-sidebar-open): Remove unused var `sidebar-window`.

* lisp/erc/erc.el: Fix header to use the customary 3 semi-colons.
(erc-fill-column): Declare variable.

* lisp/erc/erc-autoaway.el: Use lexical-binding.
* lisp/erc/erc-ezbounce.el: Use lexical-binding.
* lisp/erc/erc-fill.el: Use lexical-binding.
* lisp/erc/erc-goodies.el: Use lexical-binding.
* lisp/erc/erc-ibuffer.el: Use lexical-binding.
* lisp/erc/erc-identd.el: Use lexical-binding.
* lisp/erc/erc-join.el: Use lexical-binding.
* lisp/erc/erc-lang.el: Use lexical-binding.
* lisp/erc/erc-log.el: Use lexical-binding.
* lisp/erc/erc-menu.el: Use lexical-binding.
* lisp/erc/erc-netsplit.el: Use lexical-binding.
* lisp/erc/erc-networks.el: Use lexical-binding.
* lisp/erc/erc-pcomplete.el: Use lexical-binding.
* lisp/erc/erc-ring.el: Use lexical-binding.
* lisp/erc/erc-speedbar.el: Use lexical-binding.
* lisp/erc/erc-spelling.el: Use lexical-binding.
* lisp/erc/erc-truncate.el: Use lexical-binding.
* lisp/erc/erc-xdcc.el: Use lexical-binding.