app/testpmd: remove unnecessary cast

The list of builtin cmdline commands has unnecessary cast which
blocks compiler type checking.

Fixes: af75078 ("first public release")
Cc: stable@dpdk.org

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Ferruh Yigit <ferruh.yigit@amd.com>