bug#21563: 24.5; discourage load-hook variables

[Glenn Morris]

Stefan Kangas wrote:

> This suggests that all such variables should be considered obsolete,
> since eval-after-load is cleaner.  I think I agree, but I have only
> ever used eval-after-load so I might be missing something.
>
> I would suggest to declare the above variables obsolete and point
> users to eval-after-load instead.

I did this years ago for a few modes; eg dd210a6, 041e909, but never got
round to doing the rest. As I said in https://debbugs.gnu.org/24491#8, I
see no need to keep any *-load-hooks.