libkern.h: remove HAVE_INLINE_* macros

[mhorne]

Summary

The final consumer of this was OpenZFS, fixed in ffaea0831973 (thanks
mav@). These can now be retired.

[mav]

I don't know if anybody run older ZFS built out of tree, but if so, I wonder if it could have sense to give it some time to soak.

[mav]

I've requested removal of those constants from OpenZFS 2.3.6. In 2.4.0 they are not used already. So in few weeks there will be at least one release from each active branch not depending on them any more.

[mhorne]

In D55201#1263765, @mav wrote:

I've requested removal of those constants from OpenZFS 2.3.6. In 2.4.0 they are not used already. So in few weeks there will be at least one release from each active branch not depending on them any more.

Thank you. I am happy to wait on this removal as long as seems useful.

[mhorne]

@mav I found this review again. Seems the removal made it to the 2.3 release branch. Shall I proceed here?

[mav]

In D55201#1355103, @mhorne wrote:

@mav I found this review again. Seems the removal made it to the 2.3 release branch. Shall I proceed here?

@mhorne Yeah, I think we can do it in main.