Remove p_code from struct proc.

Contrary to the comments, it was never used by core dumps or
debuggers. Instead, it used to hold the signal code of a pending
signal, but that was replaced by the 'ksi_code' member of ksiginfo_t
when signal information was reworked in 7.0.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D20047