Drop COMPAT_10 on amd64. The support for it comes down to one ifdef in
trap.c - code that is incorrect anyway, there were originally three lcall
LDT slots, and here only one instruction is decoded.

Given that one of these slots was used by BSDi's syscall, also remove the
references to COMPAT_NOMID to make clear we don't support that (it already
is not enabled).

Note: for some reason, COMPAT_10 does not even compile, because there are
"multiple definitions of _KERNEL_OPT_COMPAT_...", and I don't really
understand where this comes from.

(maxv)