Delete kgzip and kgzldr.

[delphij]

Summary

These have been detach from build for long time.
Removing these as a part of sys/sys/inflate.h removal.

Test Plan

make buildorld TARGET_ARCH=i386

[ota_j.email.ne.jp]

Delete unneeded comments and put 2019 May.

[ota_j.email.ne.jp]

It looks this change hasn't yet committed.

Could you hold on to this for a while?

I found https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=183666 and it looks kgzip was working until 8.4 release which is much newer release than I had found. I'd like to spend more time to find how to set it and and if this still works.

[imp]

These changes look good to me.
I'm surprised it worked as late as 8.4.

[delphij]

Rebase and minor fixup.