Don't clear out local variables in `with-help-window'

Don't clear out local variables in `with-help-window'

* lisp/help-mode.el (help-mode-setup): Declare obsolete.
(help-mode-finish): Ditto.

* lisp/help.el (with-help-window): Don't be a wrapper around
`with-temp-buffer-window', because that made the macro big and
difficult to understand.
(help--window-setup): Implement the relevant bits from
`with-temp-buffer-window'.  Also don't clear out local variables,
because that makes things like `text-scale-mode' not work (bug#25979).