build: T7301: do not include the GPG image signing key in images

[dmbaturin]

Change summary

Minisign signing has been in place for a long time, so there's no need for the legacy GPG signing anymore.

Removing gnupg/gpgv from package lists doesn't have negative effects because they are installed as dependencies of APT in any case.

Companion PR: vyos/vyos-1x#4429

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe): obsolete functionality removal

Related Task(s)

Related PR(s)

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

[github-actions]

👍
No issues in PR Title / Commit Title

[sever-sever]

Removing legacy GPG signing was discussed in the maintainers' meeting.
Do not have any objection.

[c-po]

Remove legacy validation key