Reverting "ox-publish: Small refactoring"

Revert "ox-publish: Small refactoring"

This reverts commit 1d6f5b62b8af3d178c7fd041e0def8cba1e52498.

lisp/ox-publish.el

@@ -568,7 +568,9 @@ Return output file name."
 		;; filters.  It isn't dependent on `:makeindex', since
 		;; we want to keep it up-to-date in cache anyway.
 		(org-combine-plists
-		 plist '(:filter-parse-tree (org-publish-collect-index))))))
+		 plist `(:filter-parse-tree
+			 ,(cons 'org-publish-collect-index
+				(plist-get plist :filter-parse-tree)))))))
       ;; Remove opened buffer in the process.
       (unless visitingp (kill-buffer work-buffer)))))