Time to remove src/distrib/cdrom stuff?

Hi,

I've just committed changes to move macppc specific bootable installcd
stuff from src/distrib/cdrom to sys/arch/macppc/stand dirs:

 https://mail-index.netbsd.org/source-changes/2024/09/15/msg153340.html
 https://mail-index.netbsd.org/source-changes/2024/09/15/msg153341.html
 https://mail-index.netbsd.org/tech-toolchain/2024/09/15/msg004534.html

Now no file in src/distrib/cdrom is necessary on "build.sh iso-image"
processes, so I think it's time to G/C all historical src/distrib/cdrom
stuff.

The only functionality of src/distrib/cdrom not supported by build.sh
is "fetching release binaries and use them on creating ISO":
 https://nxr.netbsd.org/xref/src/distrib/cdrom/README?r=1.4

However I'm not sure it's still tested on recent releases and
it also requires optional /usr/pkg/bin/rsync binary, so I think
there is less worth to maintain such stuff.
(even github actions can handle "build.sh release iso-image" as CI tests)

If there is no objection, I'll remove all src/distrib/cdrom stuff
at least before netbsd-11 cut.

Thanks,
---
Izumi Tsutsui