Remove duplicate key definitions from keymaps

Remove duplicate key definitions from keymaps

* lisp/gnus/gnus-srvr.el (gnus-browse-mode-map): Remove binding for
'gnus-browse-prev-map'.
* lisp/gnus/gnus-sum.el (gnus-summary-mode-map): Remove
'gnus-summary-down-thread' binding, and by symmetry
'gnus-summary-up-thread' (it's still available on "T-u").
* lisp/ibuffer.el (ibuffer-mode-map): Remove binding for
'ibuffer-mark-for-delete'.
* lisp/wdired.el (wdired-perm-mode-map): Remove duplicate binding of
"s" to 'wdired-set-bit'.

* etc/NEWS: Announce 'gnus-summary-up-thread' change.