staging: comedi: tests: ni_routes_test: Remove unused variable 'olddevroutes'

staging: comedi: tests: ni_routes_test: Remove unused variable 'olddevroutes'

GCC reports the following warning with W=1:

drivers/staging/comedi/drivers/tests/ni_routes_test.c:215:45: warning:
 variable 'olddevroutes' set but not used [-Wunused-but-set-variable]
  215 |  const struct ni_device_routes *devroutes, *olddevroutes;
      |                                             ^~~~~~~~~~~~

This variable is not used in function , this commit
remove it to fix the warning.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Qiheng Lin <linqiheng@huawei.com>
Link: https://lore.kernel.org/r/20210410064932.12905-1-linqiheng@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>