sysent: retire unused SV_IA32

When COMPAT_IA32 was generaliaed to COMPAT_FREEBSD32 in 2010 (commit
841c0c7ec75b), all runtime uses of SV_IA32 were removed in favor of
SV_ILP32 check. Given SV_ILP32's existance it makes no
sense to dynamically check for SV_IA32 (even if we had a 32-bit x86 ABI
with 64-bit time_t we needed to differentiate, SV_IA32 would be the
wrong spelling.) As such, remove SV_IA32 and mark the bit reserved.

Reviewed by: kib
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D50672