Replace mode-specific quit function in ibuffer.el

Replace mode-specific quit function in ibuffer.el

* lisp/ibuffer.el: Replace mode-specific quit function with
`quit-window' via `special-mode'.
(ibuffer-mode-map): Use keybindings from special-mode-map instead
of local overrides.
(ibuffer): Don't store previous windows configuration. Let
`quit-window' handle restoring.
(ibuffer-quit): Remove function. Use `quit-window' instead.
(ibuffer-restore-window-config-on-quit): Remove variable.
(ibuffer-prev-window-config): Remove variable.