Fix unused variable warning in usb's atp.c

With clang 15, the following -Werror warning is produced:

sys/dev/usb/input/atp.c:2018:11: error: variable 'n_vertical_scrolls' set but not used [-Werror,-Wunused-but-set-variable]
        u_int8_t n_vertical_scrolls = 0;
                 ^

The 'n_vertical_scrolls' variable is no longer used after 197b9a2ef003,
so remove it.

MFC after: 3 days