Removing USE_RADIX in ip_encap.c

[Ryota Ozaki]

Hi,

It seems USE_RADIX switch in ip_encap.c is always
on for years and nobody uses non-USE_RADIX case.
(It even cannot be built w/o USE_RADIX.)

I'll remove the switch in a few days if there is
no objection.

  ozaki-r