tracing: remove unused variable

Remove the following warning with CONFIG_TRACING=y:

	kernel/trace/trace.c: In function ‘s_next’:
	kernel/trace/trace.c:1186: warning: unused variable ‘last_ent’
Signed-off-by: Andrea Righi <righi.andrea@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/trace/trace.c

@@ -1183,7 +1183,6 @@ static void *find_next_entry_inc(struct trace_iterator *iter)
 static void *s_next(struct seq_file *m, void *v, loff_t *pos)
 {
 	struct trace_iterator *iter = m->private;
-	void *last_ent = iter->ent;
 	int i = (int)*pos;
 	void *ent;