Drop i486 from the default i386 GENERIC kernel configuration.

80486 production was stopped by Intel on September 2007. Dropping the 486
configuration option from the GENERIC kernel improves performance
slightly.

Removing I486_CPU is consistent at this time: we don't support any
processor without a FPU and the PC-98 arch, which frequently involved i486
CPUs, is also gone so we don't test such platforms anymore.

Relnotes: yes
MFC after: 2 weeks
https://reviews.freebsd.org/D9879