Variable num_cols is being assigned but is never used hence it is
redundant and can be removed.

Cleans up clang warning:
warning: variable ‘num_cols’ set but not used [-Wunused-but-set-variable]
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>