remove asciinema upload from update-demos target (#2786)

asciinema.org is unreachable from GitHub Actions runners (Azure)
since June 16, 2026 (asciinema/asciinema-server#479), causing the
daily demo CI job to fail consistently. The upload step is not
essential — the CI job's purpose is to validate that demo recordings
can be generated successfully. Remove the -u flag and update the
target's EXHELP text accordingly.

Co-authored-by: Per G. da Silva <pegoncal@redhat.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>