`usesh` input is no longer used but still referenced in several files

[tsutsui]

The usesh input seems obsolete after anyvm changes:
655c939

but is still defined and referenced in several places:

https://github.com/vmactions/netbsd-vm/blob/v1/action.yml
https://github.com/vmactions/netbsd-vm/blob/v1/README.md
https://github.com/vmactions/netbsd-vm/blob/v1/.github/workflows/test.yml

If usesh is no longer supported, could you please remove all references to avoid confision.

Thanks,

[neilpang]

Confirmed, usesh has done nothing since the switch to anyvm in
655c939 -- prepare and run are always piped to sh in the guest now,
whatever the login shell of that system is.

Removed the references from the base-vm templates: the README example,
the test workflow, and the per-OS shell note. Every *-vm repo picks
that up on its next regeneration, so they disappear from netbsd-vm too.
The vmactions.org docs are updated as well.

The input itself stays declared in action.yml, marked deprecated and
ignored. Dropping the declaration would make the runner print
"Unexpected input(s) 'usesh', valid inputs are [...]" on every existing
workflow that still passes it, and the README example carried
usesh: true for years, so there are a lot of those. It costs nothing to
leave in place, and you can just delete the line.

Thanks for the report.