Remove obsolete purpose-conf argument for good

Now that emacs 25 is the minimum version supported this should not cause any more problems.

layers/+spacemacs/spacemacs-purpose/local/spacemacs-purpose-popwin/spacemacs-purpose-popwin.el

@@ -185,10 +185,7 @@ Popwin's settings are taken from `popwin:special-display-config'."
                 (cond ((symbolp pattern) mode-purposes)
                       ((plist-get settings :regexp) regexp-purposes)
                       (t name-purposes))))
-    ;; "pupo" argument is obsolete in Emacs 25, but still mandatory in 24.5 (see
-    ;; https://github.com/syl20bnr/spacemacs/issues/9583)
-    (purpose-conf "pupo"
-                  :mode-purposes mode-purposes
+    (purpose-conf :mode-purposes mode-purposes
                   :name-purposes name-purposes
                   :regexp-purposes regexp-purposes)))