ox-beamer: Fix environment selection wrt persistent tags

* lisp/ox-beamer.el (org-beamer-select-environment): Ignore persistent
  tags when displaying environments.

Thanks to Anders Johansson for reporting it.
http://permalink.gmane.org/gmane.emacs.orgmode/86441

lisp/ox-beamer.el

@@ -1092,6 +1092,7 @@ aid, but the tag does not have any semantic meaning."
 			  envs)
 		  '((:endgroup))
 		  '(("BMCOL" . ?|))))
+	 (org-tag-persistent-alist nil)
 	 (org-use-fast-tag-selection t)
 	 (org-fast-tag-selection-single-key t))
     (org-set-tags)