org-create-math-formula: Do not run file-related hooks when checking output

* lisp/org.el (org-create-math-formula): Avoid using
`find-file-noselect' to check contents of the generated mathml
formula.  `find-file-noselect' runs a number of hooks, which are not
necessary and may sometimes throw errors.

Link: https://orgmode.org/list/ut96a7$i6d$1@ciao.gmane.io

lisp/org.el

@@ -16225,7 +16225,8 @@ inspection."
     (setq shell-command-output (shell-command-to-string cmd))
     (setq mathml
 	  (when (file-readable-p tmp-out-file)
-	    (with-current-buffer (find-file-noselect tmp-out-file t)
+	    (with-temp-buffer
+              (insert-file-contents tmp-out-file)
 	      (goto-char (point-min))
 	      (when (re-search-forward
 		     (format "<math[^>]*?%s[^>]*?>\\(.\\|\n\\)*</math>"