Don't check for `cadr' and `last'.

Don't check for `cadr' and `last'.
(iswitchb-define-mode-map, iswitchb-default-keybindings):
Add obsolescence declaration and remove redundant info from docstring.
(iswitchb-set-common-completion, iswitchb-set-matches,
iswitchb-get-matched-buffers, iswitchb-visit-buffer): Use `let'.
(recentf-list, most-len, most-is-exact):
Don't wrap defvars within `eval-when-compile'.