Prevent unwanted recursion in erc-nickbar-disable

Prevent unwanted recursion in erc-nickbar-disable

* lisp/erc/erc-speedbar.el (erc-status-sidebar-mode--unhook): Remove
forward declaration.
(erc-speedbar--toggle-nicknames-sidebar): Inline
`erc-speedbar-close-nicknames-window'.  Don't call
`erc-speedbar-browser' and thus avoid adding excess timers.
(erc-speedbar--ensure): Inline `speedbar-enable-update' to avoid
unneeded call to `speedbar-set-timer', and ensure it only runs in
`speedbar-buffer'.
(erc-speedbar--shutting-down-p): New flag variable to avoid recursive
calls to `dframe-close-frame' and friends.
(erc-nickbar-mode, erc-nickbar-enable, erc-nickbar-disable): Move
logic formerly performed by `speedbar-disable-update' to
`erc-speedbar--toggle-nicknames-sidebar'.  When disabling, guard
against recursive calls to `dframe-close-frame' and friends.
(erc-speedbar--get-timers): New utility function, also for use in
testing.
(erc-speedbar--dframe-controlled): Bind
`erc-speedbar--shutting-down-p' flag non-nil around call to
`erc-nickbar-mode'.  Remove excess timer left behind due to
incompatible behavior from `dframe-close-frame'.  Let caller kill
buffer.
(erc-speedbar-close-nicknames-window): Remove unused command, new in
ERC 5.6 and Emacs 30.
* test/lisp/erc/erc-scenarios-status-sidebar.el
(erc-speedbar-close-nicknames-window): Remove forward declaration.
(erc-speedbar--get-timers): Add forward declaration.
(erc-scenarios-status-sidebar--nickbar): Fix faulty expectations of
desired behavior when disabling module.  Ensure timers canceled.
* test/lisp/erc/resources/erc-scenarios-common.el
(erc-scenarios-common--make-bindings): Shadow `timer-idle-list' to
avoid polluting global test environment with stray timers.
(Bug#63595)