Remove support for kernel.tramp and kernel.tramp.gz

Nothing uses these anymore. They were for super small armv4 boards without
uboot. We removed armv4 support before 13.0, but neglected to garbage collect
this at the same time. Today, both flavors of armv5 kernels (mv and ralink) boot
via uboot which has its own compression scheme for boards that need it.

Note: OLDFILES has not been updated beacuse installkernel will move the whole
directory out of the way before installing the new kernel.

Differential Revision: https://reviews.freebsd.org/D21072