ring-buffer: remove unused variable

Fix the compiler error:

kernel/trace/ring_buffer.c: In function 'rb_move_tail':
kernel/trace/ring_buffer.c:1236: warning: unused variable 'event'
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/ring_buffer.c

@@ -1233,7 +1233,6 @@ rb_move_tail(struct ring_buffer_per_cpu *cpu_buffer,
 {
 	struct buffer_page *next_page, *head_page, *reader_page;
 	struct ring_buffer *buffer = cpu_buffer->buffer;
-	struct ring_buffer_event *event;
 	bool lock_taken = false;
 	unsigned long flags;