Fix remove-signing-service handling, can now remove service without --class argument

[aKlimau]

Closes 7318

📜 Checklist

• Commits are cleanly separated with meaningful messages (simple features and bug fixes should be squashed to one commit)
• A changelog entry or entries has been added for any significant changes
• Follows the Pulp policy on AI Usage
• (For new features) - User documentation and test coverage has been added

[patchback]

Backport to 3.73: 💚 backport PR created

✅ Backport PR branch: patchback/backports/3.73/848f3e063204dde9f9016abe8392b5eb8e072283/pr-7445

Backported as #7448

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.

[patchback]

Backport to 3.85: 💚 backport PR created

✅ Backport PR branch: patchback/backports/3.85/848f3e063204dde9f9016abe8392b5eb8e072283/pr-7445

Backported as #7449

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.

[patchback]

Backport to 3.105: 💚 backport PR created

✅ Backport PR branch: patchback/backports/3.105/848f3e063204dde9f9016abe8392b5eb8e072283/pr-7445

Backported as #7450

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.