Revert "org-export-data: Concatenate strings in temporary buffer for performance"

This reverts commit a158b263a6.
The commit actually makes not measurable performance difference.

lisp/ox.el

@@ -1927,9 +1927,9 @@ Return a string."
 			      (and (not greaterp)
 				   (memq type org-element-recursive-objects)))
 			     (contents
-                              (let ((export-buffer (current-buffer)))
-                                (with-temp-buffer
-                                  (dolist (element (org-element-contents
+			      (mapconcat
+			       (lambda (element) (org-export-data element info))
+			       (org-element-contents
 				(if (or greaterp objectp) data
 				  ;; Elements directly containing
 				  ;; objects must have their indentation
@@ -1947,14 +1947,8 @@ Return a string."
 				    (eq (car (org-element-contents parent))
 					data)
 				    (eq (org-element-property :pre-blank parent)
-					                    0))))))
-                                    (insert
-                                     ;; Use right local variable
-                                     ;; environment if there are, for
-                                     ;; example, #+BIND variables.
-                                     (with-current-buffer export-buffer
-                                       (org-export-data element info))))
-                                  (buffer-string)))))
+					0)))))
+			       "")))
 			(broken-link-handler
 			 (funcall transcoder data
 				  (if (not greaterp) contents