[ada, v3, doc] Remove obsolete IRIX 5/6 support

[Bruce Korb]

Hi Ranier,

I didn't see any fixinclude fixes in the patch.  They must have dropped out
somewhere.  Anyway, if you're removing something that you're only
"pretty sure is dead", maybe comment it out with:

#ifdef I_THINK_THIS_IS_DEAD_AS_OF_APRIL_2010
...........
#endif

and if nobody complains, then dump it.  Maybe put a date on it and if in
some number of months there's no complaint, it would be easy to find
old dead stuff to eliminate.  :)