staging: comedi: remove unused variable 'route_table_size'

drivers/staging/comedi/drivers/ni_routes.c:52:21: warning:
 route_table_size defined but not used [-Wunused-const-variable=]

It is never used since introduction.
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://lore.kernel.org/r/20191023075206.33088-1-yuehaibing@huawei.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/drivers/ni_routes.c

@@ -49,8 +49,6 @@
 /* Helper for accessing data. */
 #define RVi(table, src, dest)	((table)[(dest) * NI_NUM_NAMES + (src)])
 
-static const size_t route_table_size = NI_NUM_NAMES * NI_NUM_NAMES;
-
 /*
  * Find the proper route_values and ni_device_routes tables for this particular
  * device.