RFC: Remove documentation of IRIX as supported platform

[Bruno Haible]

Since
  - IRIX 6.5 is end-of-life for more than 10 years [1],
  - I don't have access to an IRIX machine any more,
  - the AC_SYS_LARGEFILE macro no longer supports IRIX,
I would suggest to remove mentions of IRIX support from the documentation.
Like we did for OSF/1 in 2019:

        doc: Remove documentation of OSF/1 as supported platform.
        * doc/gnulib-intro.texi (Target Platforms): Mention that OSF/1 is
        unsupported.
        ...
        * doc/**/*.texi: Update.

Any objections?

   Bruno

[1] 
https://git.savannah.gnu.org/gitweb/?p=gnulib/maint-tools.git;a=blob;f=end-of-life.txt