kernel.tramp* to be removed

[Warner Losh]

The kernel.tramp* support was for small boards w/o uboot that needed a
compression story. These are primarily the early armv4 boards we supported,
though a few armv5 boards could use it because they were successor to
earlier armv4 boards. Today, both ralink and marvel, the only two remaining
armv5 CPUs, use uboot, which has its own compression story.

As such, this is dead code and will be removed. It is also getting in the
way of having a unified compression story between userland and the kernel,
as it's the only semi-working code that uses the old interfaces.

Since this is dead code, there will be no deprecation phase. However, it
won't be merged to 11.x. I might merge it to stable/12 if we have the same
issue when the unified compression code gets merged. Since it's dead code,
it can be removed from the branch w/o issue.

Before I pull the trigger, I thought I'd post this review:
https://reviews.freebsd.org/D21072
and make a 'last call' for people who are interested to object and show
that it's actually still in use despite my careful analysis and the word of
the closest thing we have to a marvel armv5 maintainer. Please take all
feedback to the review. Thanks!

Warner