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>