Backport: Don't autoload erc-modules

* doc/misc/erc.texi (Modules): Recommend using `describe-variable'
instead of `customize-option' because the latter needs the symbol to be
loaded.
* lisp/erc/erc.el (erc-modules): Remove autoload cookie because it
caused customizations for this option to load the main library.  This
reverts the thrust of bb894845 "Teach customize-option about
erc-modules", which was added in ERC 5.6 and Emacs 30.  The motivation
for the original offending change was to allow new users to run M-x
customize-option RET erc-modules RET immediately after startup instead
of M-x customize-group RET, followed by an I-search.  (Bug#73812)

(Cherry picked from commit 1854f2751e)

Do not merge to master.

doc/misc/erc.texi

@@ -411,8 +411,9 @@ Kill current input line using @code{erc-bol} followed by @code{kill-line}.
 One way to add functionality to ERC is to customize which of its many
 modules are loaded.
 
-There is a spiffy customize interface, which may be reached by typing
-@kbd{M-x customize-option @key{RET} erc-modules @key{RET}}.  When
+You can do this by typing @kbd{C-h v erc-modules @key{RET}} and clicking
+@samp{customize} near the bottom of the resulting help buffer, where it
+says ``You can @emph{customize} this variable.''  When
 removing a module outside of Customize, you may wish to ensure it's
 disabled by invoking its associated minor-mode toggle with a
 nonpositive prefix argument, for example, @kbd{C-u - M-x

lisp/erc/erc.el

@@ -2263,8 +2263,6 @@ buffer rather than a server buffer.")
       (cl-pushnew mod (if (get mod 'erc--module) built-in third-party)))
     `(,@(sort built-in #'string-lessp) ,@(nreverse third-party))))
 
-;;;###autoload(custom-autoload 'erc-modules "erc")
-
 (defcustom erc-modules '( autojoin button completion fill imenu irccontrols
                           list match menu move-to-prompt netsplit
                           networks readonly ring stamp track)