intel_th: msu: Remove set but not used variable 'last'

Commit aad14ad3 ("intel_th: msu: Add current window tracking") added
the following gcc warning:

> drivers/hwtracing/intel_th/msu.c: In function msc_win_switch:
> drivers/hwtracing/intel_th/msu.c:1389:21: warning: variable last set but
> not used [-Wunused-but-set-variable]

Fix it by removing the variable.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Fixes: aad14ad3 ("intel_th: msu: Add current window tracking")
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20190621161930.60785-3-alexander.shishkin@linux.intel.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>