tweaks: remove a redundant call, as there is nothing to free there

At that point, 'u->cutbuffer' will always be NULL, either from the
original creation of the INSERT undo item, or by having been freed
and set to NULL by do_redo().

src/text.c

@@ -627,7 +627,6 @@ void do_undo(void)
 		openfile->mark_x = u->mark_begin_x;
 		goto_line_posx(u->lineno, u->begin);
 		cut_marked(NULL);
-		free_lines(u->cutbuffer);
 		u->cutbuffer = cutbuffer;
 		cutbuffer = oldcutbuffer;
 		break;