mmc: mmc_test: Remove set-but-unused variable.

Fixes:

drivers/mmc/card/mmc_test.c: In function ‘mmc_test_seq_perf’:
drivers/mmc/card/mmc_test.c:1878:28: warning: variable ‘ts’ set but not
used [-Wunused-but-set-variable]

There's no reason to be calling timespec_sub() here, because
mmc_test_print_avg_rate() is going to do that itself.

Signed-off-by: Chris Ball <cjb@laptop.org>
Cc: Adrian Hunter <adrian.hunter@nokia.com>