gtk: remove begin-user-action/end-user-action around copying

Copying to the clipboard is not a buffer mutation, so calling
gtk_text_buffer_begin_user_action() and _end_user_action() is only
confusing apps which connect to these signals in order to build undo
stacks or otherwise track buffer changes. Most likely, these apps
either didn't notice the bugus undo step or simply work around it.
(cherry picked from commit 0a49865ec4)

gtk/gtktextbuffer.c

@@ -4016,9 +4016,7 @@ void
 gtk_text_buffer_copy_clipboard (GtkTextBuffer *buffer,
 				GtkClipboard  *clipboard)
 {
-  gtk_text_buffer_begin_user_action (buffer);
   cut_or_copy (buffer, clipboard, FALSE, TRUE, TRUE);
-  gtk_text_buffer_end_user_action (buffer);
 }
 
 /**